Cleanup Cell equality

As pointed out by dbrosius, it is possible for AbstractCell to generate false positive equality assertions if, e.g., a non-expiring cell is replaced by an expiring equivalent. It's not clear to me if this would ever be a real problem, since we would reconcile correctly, and as such equality would unlikely ever be a problem (it's used rarely, only really for hash maps I think (although auditing this is difficult) in which the hash code would also be different, and by which point we should have resolved conflicts). It's also a very small exposure profile, with either non-ttl'd writes mixed with ttl'd, or a delete of a value representing the deletion timestamp.