Exponential back-off does not work with multiple sites

If a cache is backup to 2 or more sites, and one site is down, the SITE_UNREACHABLE events will enable the back-off mechanism.

The problem is the ack from the other(s) sites will disable the back-off making it useless.

Also, I've noticed when the retry happens, it sends to all sites again. This can be improved and we should send the update to the missing sites only.