MINOR: mux-quic: account stream txbuf in QCC

MINOR: mux-quic: account stream txbuf in QCC

A limit per connection is put on the number of buffers allocated by QUIC
MUX for emission accross all its streams. This ensures memory
consumption remains under control. This limit is simply explained as a
count of buffers which can be concurrently allocated for each
connection.

As such, quic_conn structure was used to account currently allocated
buffers. However, a quic_conn nevers allocates new stream buffers. This
is only done at QUIC MUX layer. As such, this commit moves buffer
accounting inside QCC structure. This simplifies the API, most notably
qc_stream_buf_alloc() usage.

Note that this commit inverts the accounting. Previously, it was
initially set to 0 and increment for each allocated buffer. Now, it is
set to the maximum value and decrement for each buf usage. This is
considered as clearer to use.