Transfer bytes from the output queue of each protocol to the input of the other.
| Parameters | server | The protocol instance representing the server-side of this connection. |
| serverToClient | The _LoopbackQueue holding the server's output. |
| client | The protocol instance representing the client-side of this connection. |
| clientToServer | The _LoopbackQueue holding the client's output. |
| pumpPolicy | See loopbackAsync. |
| Returns | A Deferred which fires when the connection has been closed and both sides have received notification of this. |