CASSANDRA-5459: remove node from seeds when it completely leaves the cluster.
This version slightly refactors Gossiper by pulling th seeds list building code
out of start() and into a shared method that removeEndpoint() will call.
Also, reload the seeds list, then remove the dead node.
Remove node from seeds list when it permanently leaves the cluster
patch by jasobrown; freviewed by driftx for CASSANDRA-5459.