Ignore duplicate delete requests

Stop delete requests from queueing up in the compute manager if the
task_state is already set to DELETING. This prevents exhausting thread
pools when the service is stuck on a lock/external request.

Dealing with delete requests that never got executed is not in scope of
this change and will be submitted separately.

Change-Id: I2f97f93bd714e0ea3b6d4fa3ac457ab43eed00e1
Partial-Bug: #1248563
Merge "Ignore duplicate delete requests"