MCR: Deprecate and gut Revision class

MCR: Deprecate and gut Revision class

This is a re-submission of I4f24e7fbb68.

As a first major step towards Multi-Content-Revisions (MCR),
this patch turns the Revision class into a legacy proxy for
the new RevisionRecord and RevisionStore classes.

Backwards compatibility is maintained for all but some
rare edge cases, like constructing a completely empty
Revision object.

For more information on MCR, see
<https://www.mediawiki.org/wiki/Requests_for_comment/Multi-Content_Revisions>.

NOTE: once this is merged, verify create/delete/restore cycle on beta,
      ideally with emulated replication lag.

Bug: T174025
Change-Id: Ia4c20a91e98df0b9b14b138eb4825c55e5200384