MINOR: mux-quic: realign Tx buffer if possible

MINOR: mux-quic: realign Tx buffer if possible

A major reorganization of QUIC MUX sending has been implemented. Now
data transfer occur over a single QCS buffer. This has improve
performance but at the cost of restrictions on snd_buf. Indeed, buffer
instances are now shared from stream callback snd_buf up to quic-conn
layer.

As such, snd_buf cannot manipulate freely already present data buffer.
In particular, realign has been completely removed by the previous
patches.

This commit reintroduces a partial realign support. This is only done if
the buffer contains only unsent data, via a new MUX function
qcc_realign_stream_txbuf() which is called during snd_buf.