Add graceful shutdowns for fixed-size streams

[robgjansen]

For fixed-size streams where we know in advance what the send or recv length is going to be, we can do a graceful shutdown by sending application-layer FINs after we have read everything that the other side sent us. This helps ensure we don't close sockets before the data was actually received by the other end and lose data.

For non-fixed-sized streams that are driven only by a Markov model, the graceful shutdown procedure is skipped since the other end does not know when to expect the stream to end and where to look for the footer that contains the FIN.

[stevenengler]

I'm not very familiar with tgen, but it seems good to me.

I think there's still technically a small race condition here where a tgen connection receives a footer message, sends its own footer message, and closes the socket while its outgoing footer message is still sitting in the socket's outgoing queue. But since the number of bytes will be small, there's a very high chance that it will be received by the tor client and sent on the circuit before the socket leaves the TIME_WAIT state, and so the tgen connection will be successful.