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.

server.go

@@ -1569,7 +1569,6 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			ChainHash:          *s.cfg.ActiveNetParams.GenesisHash,
 			MinBackoff:         10 * time.Second,
 			MaxBackoff:         5 * time.Minute,
-			ForceQuitDelay:     wtclient.DefaultForceQuitDelay,
 			MaxTasksInMemQueue: cfg.WtClient.MaxTasksInMemQueue,
 		})
 		if err != nil {
@@ -1603,7 +1602,6 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			ChainHash:          *s.cfg.ActiveNetParams.GenesisHash,
 			MinBackoff:         10 * time.Second,
 			MaxBackoff:         5 * time.Minute,
-			ForceQuitDelay:     wtclient.DefaultForceQuitDelay,
 			MaxTasksInMemQueue: cfg.WtClient.MaxTasksInMemQueue,
 		})
 		if err != nil {