Post 4.x Clean up

Trunk being now 5.0 means we no longer recommend users, nor test ourselves, upgrades from version 3.11 or earlier.

There is a lot that can be cleaned up, especially in the test code, around earlier versions (including older sstable formats and MessagingService versions). Stuff that was deprecated before 4.0 can also be removed. A number of separate tickets already exist for the latter.

All non-trivial child tickets will be checked with dev@