call tor_event_free, not tor_free, on events in connection_free().
connection_free_, we do a probably-redundant call to tor_free on conn->read_event and conn->write_event.
This tor_free() is redundant (as we note in a comment) whenever we're calling
connection_free. But in one case -- the case where we're running connection_free_all -- we might get into trouble. We could free the event without first running event_del() on it.