[LockManager] Various fixes to lock managers.

* Improved handling of corrupt values in cache for MemcLockManager.
  Also improved the use of Status warnings a bit.
* Removed broken special-case handling for SH->EX lock escalation.
  Updated MysqLockManager to compensate.
* Made FSLockManager only use one handle per file, which is more
  efficient and avoids errors when escalating locks (SH->EX).
* Made lock unit tests have more useful output on failure.

Change-Id: Ib304712fa2b6b3fd02bfc1b08b6f238c771960c2