MEDIUM: quic: respect the threads assigned to a bind line

MEDIUM: quic: respect the threads assigned to a bind line

Right now the QUIC thread mapping derives the thread ID from the CID
by dividing by global.nbthread. This is a problem because this makes
QUIC work on all threads and ignores the "thread" directive on the
bind lines. In addition, only 8 bits are used, which is no more
compatible with the up to 4096 threads we may have in a configuration.

Let's modify it this way:
  - the CID now dedicates 12 bits to the thread ID
  - on output we continue to place the TID directly there.
  - on input, the value is extracted. If it corresponds to a valid
    thread number of the bind_conf, it's used as-is.
  - otherwise it's used as a rank within the current bind_conf's
    thread mask so that in the end we still get a valid thread ID
    for this bind_conf.

The extraction function now requires a bind_conf in order to get the
group and thread mask. It was better to use bind_confs now as the goal
is to make them support multiple listeners sooner or later.