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

RELEASE-NOTES-1.31

@@ -71,6 +71,10 @@ changes to languages because of Phabricator reports.
 * (T180052) Mirandese (mwl) now supports gendered NS_USER/NS_USER_TALK namespaces.
 
 === Other changes in 1.31 ===
+* Introducing multi-content-revision capability into the storage layer. For details,
+  see <https://www.mediawiki.org/wiki/Requests_for_comment/Multi-Content_Revisions>.
+* The Revision class was deprecated in favor of RevisionStore, BlobStore, and
+  RevisionRecord and its subclasses.
 * MessageBlobStore::insertMessageBlob() (deprecated in 1.27) was removed.
 * The global function wfBCP47 was renamed to LanguageCode::bcp47.
 * The global function wfBCP47 is now deprecated.
@@ -123,6 +127,9 @@ changes to languages because of Phabricator reports.
 * The Block class will no longer accept usable-but-missing usernames for
   'byText' or ->setBlocker(). Callers should either ensure the blocker exists
   locally or use a new interwiki-format username like "iw>Example".
+* The RevisionInsertComplete hook is now deprecated, use RevisionRecordInserted instead.
+  RevisionInsertComplete is still called, but the second and third parameter will always be null.
+  Hard deprecation is scheduled for 1.32.
 * The following methods that get and set ParserOutput state are deprecated.
   Callers should use the new stateless $options parameter to
   ParserOutput::getText() instead.

docs/hooks.txt

@@ -2810,14 +2810,14 @@ called after the addition of 'qunit' and MediaWiki testing resources.
   added to any module.
 &$ResourceLoader: object
 
-'RevisionInsertComplete': Called after a revision is inserted into the database.
-&$revision: the Revision
-$data: the data stored in old_text.  The meaning depends on $flags: if external
-  is set, it's the URL of the revision text in external storage; otherwise,
-  it's the revision text itself.  In either case, if gzip is set, the revision
-  text is gzipped.
-$flags: a comma-delimited list of strings representing the options used.  May
-  include: utf8 (this will always be set for new revisions); gzip; external.
+'RevisionRecordInserted': Called after a revision is inserted into the database.
+$revisionRecord: the RevisionRecord that has just been inserted.
+
+'RevisionInsertComplete': DEPRECATED! Use RevisionRecordInserted hook instead.
+Called after a revision is inserted into the database.
+$revision: the Revision
+$data: DEPRECATED! Always null!
+$flags: DEPRECATED! Always null!
 
 'SearchableNamespaces': An option to modify which namespaces are searchable.
 &$arr: Array of namespaces ($nsId => $name) which will be used.