Properly send a close message for mplex substreams

[tomaka]

When an mplex substream is dropped, we try to send a close message to the remote.
However if the current output buffer is full, the message will not get through.

[tomaka]

Some things have changed in mplex, but this issue is still there.

If we destroy a substream with destroy_substream and this substream was still open, we currently don't notify the remote. Instead we should send a Reset message.

There's still the problem that destroy_substream must return instantly, therefore we can't return NotReady, and we need to add some sort of send buffer.

[romanb]

Closed by #1769.