I consider the feature complete and committed.
Patch 3 is optional and per discussion buried upthread we generally agreed that it'd be better to replace most user visible uses of the 'xid' data type with a new epoch extended 'xid64' or similar.
Patch 4, txid_incinerate, has never been intended for commit. It's a testing tool.
Patches 1 and 2 were the key parts and thanks to Robert's helpful review, advice and edits they're committed now.
Committed, done. Yay.