MEDIUM: listeners: split the thread mask between receiver and bind_conf

MEDIUM: listeners: split the thread mask between receiver and bind_conf

With groups at some point we'll have to have distinct masks/groups in the
receiver and the bind_conf, because a single bind_conf might require to
instantiate multiple receivers (one per group).

Let's split the thread mask and group to have one for the bind_conf and
another one for the receiver while it remains easy to do. This will later
allow to use different storage for the bind_conf if needed (e.g. support
multiple groups).