Sentinel: more robust failover detection as observer.
Sentinel observers detect failover checking if a slave attached to the monitored master turns into its replication state from slave to master. However while this change may in theory only happen after a SLAVEOF NO ONE command, in practie it is very easy to reboot a slave instance with a wrong configuration that turns it into a master, especially if it was a past master before a successfull failover. This commit changes the detection policy so that if an instance goes from slave to master, but at the same time the runid has changed, we sense a reboot, and in that case we don't detect a failover at all. This commit also introduces the "reboot" sentinel event, that is logged at "warning" level (so this will trigger an admin notification). The commit also fixes a problem in the disconnect handler that assumed that the instance object always existed, that is not the case. Now we no longer assume that redisAsyncFree() will call the disconnection handler before returning.
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