Distinguish between instance cooldown after reincarnation, and time of last auto restart #11707

In #11504 we have decided that instances that are reincarnated due to sled evacuation should not have to be subjected to a cooldown period. This is due to two reasons: The instances didn't actually fail, and during an update any given instance may have to be restarted several times (up to the total number of sleds).

How things work today, we calculate the cooldown period based off of the time the instance was last auto-restarted. In #11504 we had to work around this by not recording the time of last auto-restart when an instance is being reincarnated due to sled evacuation.

Going forward, we should fix this workaround and record the beginning of a cooldown period separately from the time an instance was auto-restarted.