Use a separate thread to load plugins

[DerPopo]

Especially if there are many plugins, using a separate thread to load the plugins should speed the server up.
This requires waiting for all plugins to be loaded before firing an event and/or applying attributes to events so some events (if these are fired while the server starts up) don't get passed to the plugins or get passed once the plugins are loaded.

[gwmensink]

I've re-implemented plugin loading in the master branch. It is loosly coupled so it can easily be replaced by a multi-threaded loader