In commit 56df32ba56 (gossip: Mark node as
dead even if already left). A node liveness check is missed.
Fix it up.
Before: (mark a node down multiple times)
[Tue Dec 8 12:16:33 2015] INFO [shard 0] gossip - InetAddress 127.0.0.3 is now DOWN
[Tue Dec 8 12:16:33 2015] DEBUG [shard 0] storage_service - endpoint=127.0.0.3 on_dead
[Tue Dec 8 12:16:34 2015] INFO [shard 0] gossip - InetAddress 127.0.0.3 is now DOWN
[Tue Dec 8 12:16:34 2015] DEBUG [shard 0] storage_service - endpoint=127.0.0.3 on_dead
[Tue Dec 8 12:16:35 2015] INFO [shard 0] gossip - InetAddress 127.0.0.3 is now DOWN
[Tue Dec 8 12:16:35 2015] DEBUG [shard 0] storage_service - endpoint=127.0.0.3 on_dead
[Tue Dec 8 12:16:36 2015] INFO [shard 0] gossip - InetAddress 127.0.0.3 is now DOWN
[Tue Dec 8 12:16:36 2015] DEBUG [shard 0] storage_service - endpoint=127.0.0.3 on_dead
After: (mark a node down only one time)
[Tue Dec 8 12:28:36 2015] INFO [shard 0] gossip - InetAddress 127.0.0.3 is now DOWN
[Tue Dec 8 12:28:36 2015] DEBUG [shard 0] storage_service - endpoint=127.0.0.3 on_dead
(cherry picked from commit 5a65d8bcdd)