MAJOR: mux-quic: support multiple QCS RX buffers

MAJOR: mux-quic: support multiple QCS RX buffers

Implement support for multiple Rx buffers per QCS instances. This
requires several changes mostly in qcc_recv() / qcc_decode_qcs() which
deal with STREAM frames reception and decoding. These multiple buffers
can be stored in QCS rx.bufs tree which was introduced in an earlier
patch.

On STREAM frame reception, a buffer is retrieved from QCS bufs tree, or
allocated if necessary, based on the data starting offset. Each buffers
are aligned on bufsize for convenience. This ensures there is no overlap
between two contiguous buffers. Special care is taken when dealing with
a STREAM frame which must be splitted and stored in two contiguous
buffers.

When decoding input data, qcc_decode_qcs() is still invoked with a
single buffer as input. This requires a new while loop to ensure
decoding is performed accross multiple contiguous buffers until all data
are decoded or app stream buffer is full.

Also, after qcs_consume() has been performed, the stream Rx channel is
immediately closed if FIN was already received and QCS now contains only
a single buffer with all remaining data. This is necessary as qcc_recv()
is unable to close the Rx channel if FIN is received for a buffer
different from the current readable offset.

Note that for now stream flow-control value is still too low to fully
utilizing this new infrastructure and improve clients upload throughput.
Indeed, flow-control max-stream-data initial values are set to match
bufsize. This ensures that each QCS will use 1 buffer, or at most 2 if
data are splitted. A future patch will increase this value to unblock
this limitation.