MXS-2122: Close listener socket on destruction

Even though directly closing the socket is not very neat in the
architectural sense of things, it allows the best of both worlds: the
socket is instantly closed and is open for reuse while the listener struct
is still available as a reference.

This change needs to be revised when the listeners are refactored into
separate objects.

Updated documentation to reflect the change in behavior.

server/core/dcb.cc

@@ -1274,6 +1274,11 @@ void dcb_final_close(DCB* dcb)
                 }
             }
         }
+        else
+        {
+            // Only listeners are closed with a fd of -1
+            mxb_assert(dcb->dcb_role == DCB_ROLE_SERVICE_LISTENER);
+        }
 
         dcb->state = DCB_STATE_DISCONNECTED;
         dcb_remove_from_list(dcb);

server/core/service.cc

@@ -751,6 +751,11 @@ bool service_remove_listener(Service* service, const char* target)
             {
                 listener->listener->session->state = SESSION_STATE_LISTENER_STOPPED;
                 rval = true;
+
+                // TODO: This is not pretty but it works, revise when listeners are refactored. This is
+                // thread-safe as the listener is freed on the same thread that closes the socket.
+                close(listener->listener->fd);
+                listener->listener->fd = -1;
             }
             break;
         }