Remove unused locks and variables

Removed unused spinlocks from DCBs, sessions and the MySQL protocol
structs. They were used in a context where only one thread has access to
the structure.

Removed unused member variables from DCBs.

server/modules/routing/maxinfo/maxinfo.c

@@ -291,20 +291,15 @@ static void handleError(MXS_ROUTER     *instance,
     {
         backend_dcb->dcb_errhandle_called = true;
     }
-    spinlock_acquire(&session->ses_lock);
+
     sesstate = session->state;
     client_dcb = session->client_dcb;
 
     if (sesstate == SESSION_STATE_ROUTER_READY)
     {
         CHK_DCB(client_dcb);
-        spinlock_release(&session->ses_lock);
         client_dcb->func.write(client_dcb, gwbuf_clone(errbuf));
     }
-    else
-    {
-        spinlock_release(&session->ses_lock);
-    }
 
     /** false because connection is not available anymore */
     dcb_close(backend_dcb);