MessageCache: replace should actually replace, not reload

MessageCache: replace should actually replace, not reload

Prior to I462554b30, MessageCache::replace() did just that: it took the
existing cache and updated the one entry.

In I462554b30, the rearrangement of work into a DeferredUpdate
introduced a bug: the in-process cache was updated, but when the shared
cache was loaded later the entry was never updated in there so the
shared caches kept the old value. This was found in code review and
worked around by reloading all the messages from the database instead of
updating the existing cache.

But all that extra work reloading everything from the database causes
major slowness saving any MediaWiki-namespace page when the wiki has
many such small pages. Let's go back and fix the bug so replace() can
again replace instead of reloading everything.

Bug: T203925
Change-Id: Ife8e1bd6f143f480eb8da09b817c85aadf33a923