[SPARK-3597][Mesos] Implement `killTask`.

The MesosSchedulerBackend did not previously implement `killTask`,
resulting in an exception.

Author: Brenden Matthews <brenden@diddyinc.com>

Closes #2453 from brndnmtthws/implement-killtask and squashes the following commits:

23ddcdc [Brenden Matthews] [SPARK-3597][Mesos] Implement `killTask`.