Fix memleak in TorController [rework]

It looks like, TorController::disconnected_cb(TorControlConnection&
conn) gets called multiple times which results in multiple event_new().

Avoid this by creating the event only once in the constructore, and
deleting it only once in the destructor (thanks to Cory Fields for the
idea).

Replaces the fix by Jonas Schnelli in #7610, see discussion there.