Multi-Content Revisions/Revision Retrieval

Storage layer code dealing with revisions should also be migrated from using Revision objects to the RevisionLookup service which returns RevisionRecords.

(Code experiment: https://gerrit.wikimedia.org/r/#/c/217710)

RevisionRecord should have a "lazy" more (or implementation) that only loads slot meta-data on demand, and loads slot content only when specifically requested.

RevisionRecord should work as a replacement for the Revision class in most situations.