Gossiper#markAlive can race with Gossiper#markDead

The Gossiper class is not threadsafe and assumes all state changes happen in a single thread (the gossip stage). Gossiper#convict, however, can be called from the GossipTasks thread. This creates a race where calls to Gossiper#markAlive and Gossiper#markDead can interleave, corrupting gossip state. Gossiper#assassinateEndpoint has a similar problem, being called from the mbean server thread.