[Fix] Miscellaneous bugfixes (see detailed desc.)

* take kill limit into account when setting a player's kill count
*
properly initialize attackerArena during projectile damage handling
*
decrement timers every second rather than every tick
* don't set timer
to -1 if a round is in progress in an arena
Rewrite scoreboard handling (now displays top 10 players)
[Fix] Players can't shoot themselves... what sorcery is this?!
Add pregame countdown stage of configurable duration
Add optional delay parameter to CommandForceStart
Add startCount attribute (persisted) to Arena
Arena startCount can now be set via command
[Fix] CommandForceStart progresses arena to stage 1 before setting the timer
Lazy-activate the global timer
[Fix] Using the start-count tag updates the correct attribute
Signs for arenas in the waiting stage now include players needed to start
Logging a successful saving of an arena is now optional to prevent spam
Miscellaneous fixes and touchups
Prevent item dropping by clicking outside of an inventory
Implement god mode for players in an arena
Implement spawn shield of configurable duration
Cache all config values
Add '/oitg rename <arena> <name>' command
Rename command handler so it isn't mistaken for an OITGCommand subclass
Add '/oitg search <username>' command