objectcache: optimize WAN cache key updates during HOLDOFF_TTL

objectcache: optimize WAN cache key updates during HOLDOFF_TTL

Avoid the ADD operation spam from all threads trying to access
a tombstoned key by checking the interim value cache timestamp.
This also avoids the GET/CAS spam from threads that manage to
get the mutex. If a single thread repeatedly accesses the same
tombstoned value in rapid succession, there will significantly
less cache operation spam.

Do the same for cache updates to keys in the holdoff state
due to "check keys" or the "touchedCallback" function.

Relatedly, fix getWithSetCallback() to disregard interim values
set prior to or at the same time as the latest delete() call.
This can slightly reduce the chance of the cache being behind
replica DBs for a second. It also avoids unit test failures
were a series of deletes and cache access happen at the same
timestamp (via time injection or regular system time calls).

In addition:
* Add PASS_BY_REF flag with backwards compatibility to avoid
  bloating the signature of get()/getMulti() with the new
  tombstone information needed for the above changes.
* Avoid confusing pass-by-reference in getInterimValue() and
  fix use of incorrect $asOf parameter.
* Move some more logic into setInterimValue().
* Update some comments regarding broadcasted operations that
  were not true for the currently assumed mcrouter setup.
* Rename $cValue => $curValue and $versioned => $needsVersion
  for better readability.

Bug: T203786
Change-Id: I0eb3f9b697193d39a70dd3c0967311ad7e194f20