MINOR: xprt_qstrm: define new xprt module for QMux protocol

MINOR: xprt_qstrm: define new xprt module for QMux protocol

Define a new XPRT layer for the new QMux protocol. Its role will be to
perform the initial exchange of transport parameters.

On completion, contrary to XPRT handshake, xprt_qstrm will first init
the MUX and then removes itself. This will be necessary so that the
parameters can be retrieved by the MUX during its initialization.

This patch only declares the new xprt_qstrm along with basic operations.
Future commits will implement the proper reception/emission steps.