Use predefined packet types, add comments.
Packet checks adjustments and renaming.

* More appropriate names.
SoundDistance
* Plus wither plus dragon.
* Horizontal distance.
Prefer to synchronize adding players.

Does not seem to be strictly necessary, because the data is comparably
light-weight and the data loss on some time slices would usually be
small.

Should be cleaner, as this is a seldom event (login, logout).
Allow ConfigManager.getConfig... from any thread (copy on write).

This allows fast config getting from any thread, without need to know if
it is the main thread.
Add an insertion ordered COW version of a LinkedHashMap.
Simplify FlyingFrequency by using the cow map.
Clear registered hooks on disable. + comments + messages.