Add a STOP_SENDING frame to yamux

[sukunrt]

It's useful to signal to the peer that you have stopped reading on a stream. This only resets the read half of the stream.

QUIC has STOP_SENDING frame for this: https://www.rfc-editor.org/rfc/rfc9000.html#name-stop_sending-frames

We can consider adding something similar for yamux.

[MarcoPolo]

While we're adding frames, maybe we can add a grease bit