Decommissioned nodes can remain in gossip

This may apply to other dead states as well. Dead states should be expired after 3 days. In the case of decom we attach a timestamp to let the other nodes know when it should be expired. It has been observed that sometimes a subset of nodes in the cluster never expire the state, and through heap analysis of these nodes it is revealed that the epstate.isAlive check returns true when it should return false, which would allow the state to be evicted. This may have been affected by CASSANDRA-8336.