Allow listeners to be modified

The user should be able to add, modify and remove listeners. Due to listeners being tied to actual network sockets, the process of modifying a listener must use the existing object instead of replacing it with a new one. To allow this, a destroyed services should relinquish the ownership of their listeners and place them into a pool of already existing listeners. If a new service requires a listener which was already created, it can retrieve it from the pool of available listeners. After a service is destroyed and the listener is in the pool, if no service claims the listener it should be destroyed.