perl plugin: Exported plugin_flush*() to Perl.

perl plugin: Exported plugin_flush*() to Perl.

This adds the following functions to collectd's Perl API which flush the
given plugins using the given interval.

 * Collectd::plugin_flush:
   This function is a frontend to _flush_one() and _flush_all() and
   expects up to two named parameters:
   - timeout => $timeout
   - name    => $name or [ $name1, $name2, ... ]
 * Collectd::plugin_flush_one:
   This function expects exactly two parameters, namely the timeout and
   the plugin name.
 * Collectd::plugin_flush_all:
   This function expects a single parameter, namely the timeout.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>