MINOR: quic: do not use quic_newcid_from_hash64 on BE side

MINOR: quic: do not use quic_newcid_from_hash64 on BE side

quic_newcid_from_hash64 is an external callback. If defined, it serves
as a CID method generation, as an alternative to the default random
implementation.

This mechanism was not correctly implemented on the backend side.
Indeed, <hash64> quic_conn member is only setted for frontend
connections. The simplest solution would be to properly define it also
for backend ones. However, quic_newcid_from_hash64 derivation is really
only useful for the frontend side for now. Thus, this patch disables
using it on the backend side in favor of the default random generator.

To implement this, quic_cid_generate() is splitted in two functions, for
both methods of CIDs generation. This is the responsibility of the
caller to select the proper method. On backend side, only random
implementation is now used.