objectcache: cleanup tombstone/mutex logic in doGetWithSetCallback()

objectcache: cleanup tombstone/mutex logic in doGetWithSetCallback()

This largely follows-up 6b2f13b055c84d.

* Make the comments and use of temporary variables easier to follow.
* Simplified some conditionals by remove redundant checks.
* Bypass the final set() call if $isTombstone is true. It will almost
  always be rejected due to the tombstone still being there anyway, so
  there is no point in the roundtrips. Also, the most likely case where
  it would succeed is due to the callback taking a long time to run, in
  which case data from the replication lag uncertainty period that the
  tombstone represents would be getting saved to cache with the full
  nominal TTL, which is wrong.

Change-Id: Ic28e15b24f39e128bd72ad4d905edb852bc907aa