wtclient: replay pending tasks on sessionQueue stop

This commit does a few things:
- First, it gives the sessionQueue access to the TowerClient task
  pipeline so that it can replay backup tasks onto the pipeline on Stop.
- Given that the above is done, the ForceQuit functionality of the
  sessionQueue and TowerClient can be removed.
- The bug demonstrated in a prior commit is now fixed due to the above
  changes.

htlcswitch/interfaces.go

@@ -252,9 +252,8 @@ type TowerClient interface {
 
 	// BackupState initiates a request to back up a particular revoked
 	// state. If the method returns nil, the backup is guaranteed to be
-	// successful unless the tower is unavailable and client is force quit,
-	// or the justice transaction would create dust outputs when trying to
-	// abide by the negotiated policy.
+	// successful unless the justice transaction would create dust outputs
+	// when trying to abide by the negotiated policy.
 	BackupState(chanID *lnwire.ChannelID, stateNum uint64) error
 }