BUG/MEDIUM: quic: Code sanitization about acknowledgements requirements

BUG/MEDIUM: quic: Code sanitization about acknowledgements requirements

qc_may_build_pkt() has been modified several times regardless of the conditions
the functions it is supposed to allow to send packets (qc_build_pkt()/qc_do_build_pkt())
really use to finally send packets just after having received others, leading
to contraditions and possible very long loops sending empty packets (PADDING only packets)
because qc_may_build_pkt() could allow qc_build_pkt()/qc_do_build_pkt to build packet,
and the latter did nothing except sending PADDING frames, because from its point
of view they had nothing to send.

For now on, this is the job of qc_may_build_pkt() to decide to if there is
packets to send just after having received others AND to provide this information
to the qc_build_pkt()/qc_do_build_pkt()

Note that the unique case where the acknowledgements are completely ignored is
when the endpoint must probe. But at least this is when sending at most two datagrams!

This commit also fixes the issue reported by Willy about a very low throughput
performance when the client serialized its requests.

Must be backported to 2.7 and 2.6.