MINOR: xprt_qstrm: implement Rx buffering

MINOR: xprt_qstrm: implement Rx buffering

Implement buffering for reception on xprt_qstrm layer. This is necessary
to handle reception of a truncated QMux transport parameters frame.

This is performed via a new dedicated <rxbuf> member in xprt_qstrm
context. Read is performed by reusing the buffer until a whole frame can
be read.