Stop running twice destination usability tests
Every time that a relay is measured, it is first checked whether destination is usable, and then run connect_to_destination_over_circuit, which checks again if destination is "usable". It implies two locks, and a destination can fail in a row due to the relay being unstable. It'd be better to determine that a destination is not usable, either by trying to make a connection not over Tor, or after a X number of failures in a row.