Avoid MessageCache rebuilds if replace() was called recently

Avoid MessageCache rebuilds if replace() was called recently

* If replace() was called recently, then we know that this is the
  master data center and that the messages are up-to-date. With this
  change, replace() calls have nothing to contend with aside from other
  replace() calls. Even if there were timeouts due to such contention,
  caused by high MediaWiki: page edit rates, the replace() updates would
  pick up the prior changes in passing since they do load().
* This also avoids the following scenario:
  a) Someone edits a message page, triggering replace()
  b) Some page view causes load() to trigger loadFromDB()
     due to the hash being seen as volatile due to replace()
  c) The loadFromDB() loads stale slave data and undoes
     the message key update the replace() did

Change-Id: I9cdf7ad3d67f168fcba7f633af9e32a8d1fa928e