Fix: Server does not stop

[devreluije]

Problem: Server doesn't stop when using MinecraftServer#stopCleanly inside a command

Resolves #2096

Why this bug happens ?
During the server lifecycle commands are executed in connection().tick(msTime) (ServerProcessImpl.TickerImpl class)

When you use the /shutdown command, MinecraftServer#stopCleanly method is called which stops all TickThread
After all commands are executed, serverTick(msTime) is called which start (stopped TickThread) and wait until threads are finished

Solution: set a flag to true when stop method is called and check this flag at end of tick

[thebadlorax]

i concur, when i made my /stop command i had to just put a System.exit(0) at the end.