perl plugin, Collectd.pm: Added support for flushing of specific identifiers.

perl plugin, Collectd.pm: Added support for flushing of specific identifiers.

plugin_flush() now accepts three parameters, 'timeout', 'plugins' and
'identifiers'. The meaning of the former two did not change. 'identifiers' may
be a string or an array of strings specifying identifiers that are to be
passed along to the plugins' flush callbacks. This brings the Perl API in sync
with the C API.

plugin_flush_one() and plugin_flush_all() have been re-implemented in plain
Perl and marked as deprecated. A new XSUB, Collectd::_plugin_flush, has been
added which provides a small wrapper around the C implementation of
plugin_flush().

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