core: avoid initialisation blocking when a long callback is added to backend.initialised (could prevent sessions starting)

src/memory/memory.py

@@ -355,7 +355,10 @@ class Memory(object):
             auth_d.addCallback(doStartSession)
             return auth_d
 
-        return self.host.initialised.addCallback(backendInitialised)
+        if self.host.initialised.called:
+            return defer.succeed(None).addCallback(backendInitialised)
+        else:
+            return self.host.initialised.addCallback(backendInitialised)
 
     def stopSession(self, profile):
         """Delete a profile session