Warn against caching instances of plugin class
This links eventually to https://github.com/guusdk/openfire-pushnotification-plugin/commit/48b408ca233f812c57ccf1316ce9441f521cc630
Caching custom instances has ClassLoader problems when reloading plugins in a cluster.
Raise a warning every time we see (the first?) instance of a class loaded by a PluginClassLoader being added to a cache.
Ideally, if there's a reflection or something we can go at boot time, that'd be better.
If there's something we can add as a deprecation, or a plugin compile-time warning, better still.
(Separate tickets required to fix this in our plugins)