Introduce and use MessageGroups::recache

MessageGroups::clearCache has two downsides:

• The cache will be repopulated by a random thread, which on high traffic sites is unlikely to see the changes done by the thread clearing the cache
• Multiple threads may start repopulating the cache simultaneously, causing unnecessary extra load

Replaced clearCache with the new recache to update the cache immediately
with fresh data (except in tests). A small race condition still
remains if multiple threads are doing changes at the same time,
some of the changes might be ignored until next recache happens.

Removed a call in TranslatablePage, it's not its business to deal
with the cache. I checked that all callers of addMarkedTag do clear
the cache.

Change-Id: Ide74a8ba57dcf23d2c86cd7e5fa6ce11e1f8c414