This commit adds more structure to the logging done when loading and unloading
plugins.

The plugin manager was adapted to create log statements for loading and unloading
each plugin. It also logs to std-out for log statements of priority INFO and above.
This mimics what some (but not all) plugins were doing internally.

There's an added log statement that informs that the loading of the entire set of
plugins has finished (which helps users to determine when Openfire is ready to be
used).

Corresponding log statements from individual plugins have been removed.