plugin: Added support for reconfiguring the system on the fly.

plugin: Added support for reconfiguring the system on the fly.

Reconfiguring all plugins happens in multiple steps:

First, call sdb_plugin_reconfigure_init() to deconfigure all plugins by
calling their config callbacks with a NULL config tree and unregistering all
registered callbacks.

Then, sdb_plugin_configure() and other functions may be used to provide the
new configuration or load new plugins. For all plugins which were already
loaded before, sdb_module_init() will be called with a NULL argument when
reloading them.

Finally, sdb_plugin_reconfigure_finish() will clean up leftover pieces, like
unloading plugins which are no longer in use.