release the Negotiation instance after connection establishment

[Brian Warner]

As mentioned in #173, the (large) Negotiation instance should be deallocated after negotiation is complete. The fix for #173 broke this (if it actually worked before): the Negotiation object will live as long as the connection stays up.

The fix will probably be to introduce a new, smaller Protocol (named FoolscapProtocol) which lives as long as the connection. It will start by delegating inbound .dataReceived/.connectionLost messages to the Negotiation instance, then later switch to send them to the Banana instance (releasing the Negotiation).