MEDIUM: listener: rework thread assignment to consider all groups

MEDIUM: listener: rework thread assignment to consider all groups

Till now threads were assigned in listener_accept() to other threads of
the same group only, using a single group mask. Now that we have all the
relevant info (array of listeners of the same shard), we can spread the
thr_idx to cover all assigned groups. The thread indexes now contain the
group number in their upper bits, and the indexes run over te whole list
of threads, all groups included.

One particular subtlety here is that switching to a thread from another
group also means switching the group, hence the listener. As such, when
changing the group we need to update the connection's owner to point to
the listener of the same shard that is bound to the target group.