MINOR: protocol: define new callback set_affinity

MINOR: protocol: define new callback set_affinity

Define a new protocol callback set_affinity. This function is used
during listener_accept() to notify about a rebind on a new thread just
before pushing the connection on the selected thread queue. If the
callback fails, accept is done locally.

This change will be useful for protocols with state allocated before
accept is done. For the moment, only QUIC protocol is concerned. This
will allow to rebind the quic_conn to a new thread depending on its
load.

This should be backported up to 2.7 after a period of observation.