MINOR: quic: Code factorization (TX buffer reuse)

MINOR: quic: Code factorization (TX buffer reuse)

Add qc_may_reuse_cbuf() function used by qc_prep_pkts() and qc_prep_app_pkts().
Simplification of the factorized section code: there is no need to check there
is enough room to mark the end of the data in the TX buf. This is done by
the callers (qc_prep_pkts() and qc_prep_app_pkts()). Add a diagram to explain
the conditions which must be verified to be able to reuse a cbuf struct.

This should improve the QUIC stack implementation maintenability.