MINOR: mux-quic: move qcs stream member into tx inner struct

MINOR: mux-quic: move qcs stream member into tx inner struct

Move <stream> field from qcs type into the inner structure 'tx'. This
change is only a minor refactoring without any impact. It is cleaner as
Rx buffer elements are already present in 'rx' inner structure.

This reorganization is performed before introducing of a new Tx buffer
field used for QMux protocol.