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 legacy/trac#15775 (moved) / legacy/trac#15228 (moved)) when the clients are switched off.

Connections to authorities are a priority for this change, then directory servers.