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        The advantage of this approach is that transactions work as state is
        preserved.

        Example:

           $dbh = new AnyEvent::DBI
                     "DBI:mysql:test;mysql_read_default_file=/root/.my.cnf", "", "";

        Additional key-value pairs can be used to adjust behaviour:

        on_error => $callback->($dbh, $filename, $line, $fatal)
            When an error occurs, then this callback will be invoked. On
            entry, $@ is set to the error message. $filename and $line is
            where the original request was submitted.

            If the fatal argument is true then the database connection is
            shut down and your database handle became invalid. In addition
            to invoking the "on_error" callback, all of your queued request
            callbacks are called without only the $dbh argument.

            If omitted, then "die" will be called on any errors, fatal or
            not.

        on_connect => $callback->($dbh[, $success])
            If you supply an "on_connect" callback, then this callback will
            be invoked after the database connect attempt. If the connection
            succeeds, $success is true, otherwise it is missing and $@
            contains the $DBI::errstr.

            Regardless of whether "on_connect" is supplied, connect errors
            will result in "on_error" being called. However, if no
            "on_connect" callback is supplied, then connection errors are
            considered fatal. The client will "die" and the "on_error"
            callback will be called with $fatal true.

            When on_connect is supplied, connect error are not fatal and
            AnyEvent::DBI will not "die". You still cannot, however, use the
            $dbh object you received from "new" to make requests.

        fork_template => $AnyEvent::Fork-object
            "AnyEvent::DBI" uses "AnyEvent::Fork->new" to create the
            database slave, which in turn either "exec"'s a new process
            (similar to the old "exec_server" constructor argument) or uses
            a process forked early (see AnyEvent::Fork::Early).

            With this argument you can provide your own fork template. This
            can be useful if you create a lot of "AnyEvent::DBI" handles and
            want to save memory (And speed up startup) by not having to load
            "AnyEvent::DBI" again and again into your child processes:

               my $template = AnyEvent::Fork
                  ->new                               # create new template
                  ->require ("AnyEvent::DBI::Slave"); # preload AnyEvent::DBI::Slave module

               for (...) {
                  $dbh = new AnyEvent::DBI ...
                     fork_template => $template;

        timeout => seconds
            If you supply a timeout parameter (fractional values are
            supported), then a timer is started any time the DBI handle
            expects a response from the server. This includes connection
            setup as well as requests made to the backend. The timeout spans
            the duration from the moment the first data is written (or
            queued to be written) until all expected responses are returned,
            but is postponed for "timeout" seconds each time more data is
            returned from the server. If the timer ever goes off then a
            fatal error is generated. If you have an "on_error" handler
            installed, then it will be called, otherwise your program will
            die().

            When altering your databases with timeouts it is wise to use
            transactions. If you quit due to timeout while performing
            insert, update or schema-altering commands you can end up not
            knowing if the action was submitted to the database,
            complicating recovery.

            Timeout errors are always fatal.

        Any additional key-value pairs will be rolled into a hash reference
        and passed as the final argument to the "DBI->connect (...)" call.
        For example, to suppress errors on STDERR and send them instead to
        an AnyEvent::Handle you could do:

           $dbh = new AnyEvent::DBI
                      "DBI:mysql:test;mysql_read_default_file=/root/.my.cnf", "", "",
                      PrintError => 0,
                      on_error   => sub {
                         $log_handle->push_write ("DBI Error: $@ at $_[1]:$_[2]\n");
                      };

    $dbh->on_error ($cb->($dbh, $filename, $line, $fatal))
        Sets (or clears, with "undef") the "on_error" handler.

    $dbh->timeout ($seconds)
        Sets (or clears, with "undef") the database timeout. Useful to
        extend the timeout when you are about to make a really long query.

    $dbh->attr ($attr_name[, $attr_value], $cb->($dbh, $new_value))
        An accessor for the database handle attributes, such as
        "AutoCommit", "RaiseError", "PrintError" and so on. If you provide
        an $attr_value (which might be "undef"), then the given attribute
        will be set to that value.

        The callback will be passed the database handle and the attribute's
        value if successful.

        If an error occurs and the "on_error" callback returns, then only
        $dbh will be passed and $@ contains the error message.

    $dbh->exec ("statement", @args, $cb->($dbh, \@rows, $rv))
        Executes the given SQL statement with placeholders replaced by
        @args. The statement will be prepared and cached on the server side,
        so using placeholders is extremely important.

        The callback will be called with a weakened AnyEvent::DBI object as
        the first argument and the result of "fetchall_arrayref" as (or
        "undef" if the statement wasn't a select statement) as the second
        argument.

        Third argument is the return value from the "DBI->execute" method
        call.

        If an error occurs and the "on_error" callback returns, then only
        $dbh will be passed and $@ contains the error message.

    $dbh->stattr ($attr_name, $cb->($dbh, $value))