CLEANUP: mux-quic: clean up app ops callback definitions

CLEANUP: mux-quic: clean up app ops callback definitions

qcc_app_ops is a set of callbacks used to unify application protocol
running over QUIC. This commit introduces some changes to clarify its
API :
* write simple comment to reflect each callback purpose
* rename decode_qcs to rcv_buf as this name is more common and is
  similar to already existing snd_buf
* finalize is moved up as it is used during connection init stage

All these changes are ported to HTTP/3 layer. Also function comments
have been extended to highlight HTTP/3 special characteristics.