MINOR: xprt_qstrm: implement reception of transport parameters

MINOR: xprt_qstrm: implement reception of transport parameters

Extend xprt_qstrm to implement the reception of QMux transport
parameters. This is performed via conn_recv_qstrm() which relies on the
lower xprt rcv_buf operation. Once received, parameters are kept in
xprt_qstrm context, so that the MUX can retrieve them on init.

For the reception of parameters to be active, the connection must first
be flagged with CO_FL_QSTRM_RECV.