Vanchi/better presence

* Creates a Presence Sentry (one per server instance) which can send/listen to server keep-alive and notify if a server is down.
* Eliminates need for Local/Remote managers
* Disconnect queueing is a simple timeout

/cc @zendesk/zendesk-radar


### Steps to merge
 - [ ] :+1: of the team
 - [x] Fix/rewrite failing tests
 - [x] Create multiple server tests (better remote tests)

### References
 - Jira link: https://zendesk.atlassian.net/browse/RADAR-306

### Risks
 - Presence does not work as expected.
Actually, no. All redis reply entries should get processed immediately. There should possibly no processing of incoming redis messages in between. (Because during sync we silence the client)