Cleanup event handling

I suggest that we really clean up the event handling, as done here.

Event handling needs to be callback based, so we can don't need to handle logic for each and every event type in the event library.

This have some implications for broker modules:
 - The event queue-related events is no longer available. Only affected broker module I know about is mod_gearman, which uses the check reaper event for some internal timed events. Suggested fix is to schedule a custom event for that feature instead.
 - nagios_squeue is removed from the public API, and unscheduling an event is done without accessing nagios_squeue. This affects merlin, which just needs to be updated with the new function call instead.
 - Some methods that shouldn't be exposed is no longer exposed, but those doesn't seem to affect any known broker modules anyway