Vanchi/use redis sentinel

* Use new redis sentinel library
* Get the master from the sentinels and connect to it directly
* Crash when master fails.
* Capable of restarting correctly after master-reelection.

/cc @zendesk/zendesk-radar

### Steps to merge
 - [ ] :+1: of the team


### References
 - Jira link: https://zendesk.atlassian.net/browse/RADAR-354

### Risks
 - Connection to redis/sentinel does not work. 
Actually this is moot. The redis client throws out errors if there is no error message listener on it. I would rather keep it this way.. Having an error listener on the clients is a bit risky since we want to fail hard when master dies.