Do not publish offline to redis on sentry down

Since every server who detects sentry down will publish this
in close succession, there are a lot of wasteful redis messages due to
sentry down. Instead, we simply treat it like a fake implicit offline
message arrived.
Note: The redis entry will be cleaned up later whenever
someone does a sync or a get. This may cause some client offlines to
arrive on sync from left over sentries.

/cc @zendesk/zendesk-radar


### Steps to merge
 - [ ] :+1: of the team

### References
 - Jira link: 

### Risks
 - Low: Presence issues when sentry goes down.