MINOR: quic: split CID alloc/generation function

MINOR: quic: split CID alloc/generation function

Split new_quic_cid() function into multiple ones. This patch should not
introduce any visible change. The objective is to render CID allocation
and generation more modular.

The first advantage of this patch is to bring code simplication. In
particular, conn CID sequence number increment and insertion into
connection tree is simpler than before. Another improvment is also that
errors could now be handled easier at each different steps of the CID
init.

This patch is a prerequisite for the fix on CID collision, thus it must
be backported prior to it to every affected version.