Rewritten $wgAdaptiveMessageCache

Rewritten $wgAdaptiveMessageCache

This was developed for translatewiki.net, which has but
millions of messages in MediaWiki namespace. To avoid
exploding the message cache, not all of them can be
loaded. Instead of logging what messages have been
requested, now it uses the fact that if the
message has not been customised in the
site language, there is most likely
no reason to load it in any other
language either.

Change-Id: I6dd81b3858acfd2b73332e46668d46015e99c748