[MC-84842] SMP: RCON Runnable References

I use a lot of external scripts with RCON. Apparently, there is a boolean set in the RCON runnable that is always true. Because it is always true, these runnables are never removed, even when the related threads have died.

Apparently, these instances very slowly build up.

Plumbr Analysis from 15w31c:

The memory leak detected is caused by a programming error.
The following information indicates the location of the leak in the source code
At the time the leak was detected, 415 objects existed of class
    mp
These objects had been created at
    mq.run():67
and were being held
        in value of java.util.HashMap$Node
  in table of java.util.HashMap
    in m of mq
      in target of java.lang.Thread