watchtower: allow removal during session negotiation

In this commit, the bug demonstrated in the previous commit is fixed.
The locking capabilities of the AddressIterator are used to lock
addresses if they are being used for session negotiation. So now, when a
request comes through to remove a tower address then a check is first
done to ensure that the address is not currently in use. If it is not,
then the request can go through.

watchtower/wtclient/candidate_iterator.go

@@ -155,6 +155,10 @@ func (t *towerListIterator) RemoveCandidate(candidate wtdb.TowerID,
 			return err
 		}
 	} else {
+		if tower.Addresses.HasLocked() {
+			return ErrAddrInUse
+		}
+
 		delete(t.candidates, candidate)
 	}