Re-namespace RevisionStore and RevisionRecord classes

Re-namespace RevisionStore and RevisionRecord classes

During development a lot of classes were placed in MediaWiki\Storage\.
The precedent set would mean that every class relating to something
stored in a database table, plus all related value classes and such,
would go into that namespace.

Let's put them into MediaWiki\Revision\ instead. Then future classes
related to the 'page' table can go into MediaWiki\Page\, future classes
related to the 'user' table can go into MediaWiki\User\, and so on.

Note I didn't move DerivedPageDataUpdater, PageUpdateException,
PageUpdater, or RevisionSlotsUpdate in this patch. If these are kept
long-term, they probably belong in MediaWiki\Page\ or MediaWiki\Edit\
instead.

Bug: T204158
Change-Id: I16bea8927566a3c73c07e4f4afb3537e05aa04a5