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. 
Here and below, I am simply throwing an error because we may have more than one ready listeners. (Also to keep it simpler). If you have any easy ideas on how to propagate the error above using tracker nicely, let me know.