Experiment using more specific channels

[exabrial]

Right now, redex-sm broadcasts all session eviction and destruction events to all nodes. This is well tested and the scalability is proven in production, it's possible that having the sessionManager only subscribe to events for sessions it has in memory could allow even further extreme scaling. This theory would need to be tested.

Known constraints:

• Right now, the sessionManager starts a thread for the eviction and destruction listeners. Starting 2 threads per session will cause a problem. We would likely need an async selector pattern of some sort.
• Does Jedis allocate a connection for every listener? If so, this could be a problem too.