Early soft concensus (RFC 144)

[burdges]

We've two obvious flavors of polkadot-fellows/RFCs#144 that make sense in JAM, which in essence moves the relay chain fork choice rule forward into the availability votes.

• Availability votes have a preferred_fork flag - Improves performance under relay chain equivocations, which probably doesn't matter. Enables forkful block production, which maybe useful in debugging, but could be done in other ways by running adversarial tests where relay chain equivocations.

• Availability votes imply the fork being preferred - We must be stricter on when nodes send availability votes and shash validators for sending availability votes on two+ relay chain equivocations (like we'd do with preferred_fork set on two+ equivocations). Requires a unique block producer, so we can run JAM on Aura or Sassafras, but not on Babe. Simpler overall.