MEDIUM: mux-quic: rationalize tx buffers between qcc/qcs

MEDIUM: mux-quic: rationalize tx buffers between qcc/qcs

Remove the tx mux ring buffers in qcs, which should be in the qcc. For
the moment, use a simple architecture with 2 simple tx buffers in the
qcs.

The first buffer is used by the h3 layer to prepare the data. The mux
send operation transfer these into the 2nd buffer named xprt_buf. This
buffer is only freed when an ACK has been received.

This architecture is functional but not optimal for two reasons :
- it won't limit the buffer usage by connection
- each transfer on a new stream requires an allocation