Do not publish offline to redis on sentry down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.
Merge pull request #110 from zendesk/vanchi/handle_sentry_down_without_redis_publish

Do not publish offline to redis on sentry down