collectd, plugin: Added support for "flush" callbacks.
A flush callback may be provided to make it possible to flush internal caches
(e.g. the rrdtool plugin's data cache) from outside the plugin. On SIGUSR1,
flush callback is invoked for all plugins. As flushing large amounts of data
might take some time a new thread is started to handle the request
asynchronously.
Thanks to Stefan Völkel for proposing this.
Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>