Make tor connection failures random-exponential-backoff
Every connection request in tor could be modified to random-exponential-backoff on failure.
This would resolve repeated-connection overloading issues in general.
In particular, it would reduce the risk that old versions would DoS the authorities (or fallback directories, see #15775 (moved) / #15228 (moved)) when the clients are switched off.
Connections to authorities are a priority for this change, then directory servers.