Add explicit rollback messages through the system

[SupernaviX]

Add a specific new Rollback message, written to every topic which is carrying state changes. Most consumers should propagate this message downstream and otherwise ignore it. The rollback status on blockinfo should keep the semantics it has today.

[sandtreader]

To allow for a future Commit message on the same channel, make this a StateTransitionMessage with a enum with Rollback as the only current option.