MEDIUM: mux-quic: simplify sending API

MEDIUM: mux-quic: simplify sending API

The previous commit was a major rework for QUIC MUX sending process.
Following this, this patch cleans up a few elements that remains but can
be removed as they are duplicated.

Of notable changes, offset fields from QCS and QCC are removed. They are
both equivalent to flow control soft offsets.

A new function qcs_prep_bytes() is implemented. Its purpose is to return
the count of prepared data bytes not yet sent. It also replaces
qcs_need_sending().