Support swingstore rollback

[mhofman]

What is the Problem Being Solved?

If a node encounters a divergence, resulting in a consensus failure, there is currently no way to rollback the JS state that may have caused the divergence.

agd supports a rollback command, but that only affects the cosmos state. If the cosmos state is rolled back by one block, the handover inconsistency logic will just kick in, and cosmic-swingset will simply replay the chain sends of the last block.

Description of the Design

Maybe something based on an SQLite undo log.

An alternative would be to abuse the WAL system.

Security Considerations

None

Scaling Considerations

How many blocks should the rollback window be?

Impact on #6447 ?

Test Plan

TBD

[mhofman]

Closing in favor of #7963