Translation memory changes part 2

!! All message indexes need to be rebuild !!

Has some sprawling changes because I needed to fix failing tests.
Handle tm updating properly as described in
https://www.mediawiki.org/wiki/Help:Extension:Translate/Translation_memories#Solr_backend

* MessageGroups: fix cache clearing on group deletion
* MockFileBasedMessageGroup: above fixed exposed bugs here, which caused MessageGroupsTest to fail
* ApiQueryMessageTranslations: handle no results
* DatabaseTTMServer: cleanup unused code
* MessageIndex: added hook needed for ttmserver updating
* MessageIndex: in-process cache was not updated on store

Change-Id: I8ec094df0e4ebe7a997db6134f8926981a896224
Merge "Translation memory changes part 2"