MINOR: conn_stream: add cs_send() as a default snd_buf() function

MINOR: conn_stream: add cs_send() as a default snd_buf() function

This function is generic and is able to automatically transfer data from a
buffer to the conn_stream's tx buffer. It does this automatically if the mux
doesn't define another snd_buf() function.

It cannot yet be used as-is with the conn_stream's txbuf without risking to
lose data on close since conn_streams need to be orphaned for this.