Crash when persistence connect times out

When Persistence does not *connect* within a configurable timeout period, either in redis or in redis-sentinel, Persistence should crash.  (The timeout is needed as sentinel smothers exceptions and emits at present.)

/cc @zendesk/zendesk-radar

### Steps to merge
 - [ ] :+1: of the team

### References
 - Jira link: https://zendesk.atlassian.net/browse/RADAR-437

### Risks
 - Low: unexpected crashes due to introduction of timeout
Same situation as above. 
Maybe address this situation a bit more systematically: 
. Test against lack of sentinels first to see what happens.
. Test against sentinels running, but no redis.
... (Maybe there should be a redis slave only mode here)
. Test against both sentinels and redis running.