* make subnet cycling more robust; use one stability subnet/validator; explicitly represent gossip enabled/disabled
* fix asymmetry in _snappy being used for subscriptions but not unsubscriptions
* remove redundant comment
* minimal RPC and VC support for infoming BN of subnets
* create and verify slot signatures in RPC interface and VC
* loosen old slot check
* because Slot + uint64 works but uint64 + Slot doesn't
* document assumptions for head state use; don't clear stability subnets; guard against VC not having checked an epoch ahead, fixing a crash; clarify unsigned comparison
* revert unsub fix
afbaa36ef7