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);