Add ArenaKillEvent

This event is fired whenever an arena player kills either a monster
or another player. Useful for plugin extensions.
Fix multi-arena join/spec interleaving issues.

- When a player attempts to join or spec an arena while they are
in an arena (arena or lobby), they will be denied access.
- When a player attempts to join or spec an arena while they are
spectating an arena, they will be force-left first.
Include arena in ArenaKillEvent.
Add ArenaCompleteEvent

This event is fired when the final wave was completed and contains
a set of all the players who survived until the end.
ACTUALLY add the event >_>
[Ant] Add remote dist target.
Some null checks for pet spawner.