Add RCCacheEntryBuilder, split from EnhancedChangesList and cleaned up

This makes it more feasible for Wikibase, Flow, etc. to support
enhanced changes format, and allow better support for the rc_source
column in the future.

Change-Id: I873f6b86007000a94337f0c963df4bf8fec5b715
Merge "Add RCCacheEntryBuilder, split from EnhancedChangesList and cleaned up"