Dyad forcibly closes sockets on shutdown

[fatcerberus]

When dyad_shutdown() is called, all outstanding streams have dyad_close() called on them. This prevents any data recently written to the stream--for example, a message indicating the session is ending--not to be received on the other end. It would be nicer if instead, shutdown could flush write buffers before closing the streams.

This could be optional, as the current behavior may be preferable in some circumstances.