Deprecate CronCollisionPolicy.RUN_OVERLAP.

To prevent cluster starvation, the scheduler now ignores new cron jobs
with RUN_OVERLAP and behaves like CANCEL_NEW for existing ones. The
choice of CANCEL_NEW was arbitrary but IMO fits with the mental
preemption model of Aurora better (you're never guaranteed that a new
instance will schedule but Aurora doesn't tend to kill existing
instances outside of an established preemption routine).

Testing Done:
./gradlew build

Bugs closed: AURORA-38

Reviewed at https://reviews.apache.org/r/16874/