Add comments.
Add JoinLeaveListener component.
Remove angle data on players leaving.
[BLEEDING, INSTABLE] Switch horizontal velocity to a new method.

This method remembers (currently) each velocity added and only uses it
when it is needed not to generate a violation. Further (currently
simple) activation and invalidation logics are applied. With this method
latency will be much less of a problem, though stacking of queued
entries (especially for rocket-boots style flying) and more
merging of entries and more invalidation logics are required, thus
bleeding+instable.

On the long term this should make cheating much more difficult, possible
steps are:
1. Use method for vertical velocity too (only positive)
2. Distinguish positive and negative vertical velocity (opens a way to
control the speed downwards in any medium!).
3. Per-axis velocity (either absolute or pos/neg with more invalidation
logic on direction changes).