collect headers in preparation to multicast

Signed-off-by: Mattias Andrée <maandree@operamail.com>
get multicast recipients

Signed-off-by: Mattias Andrée <maandree@operamail.com>
sort interceptors by priority

Signed-off-by: Mattias Andrée <maandree@operamail.com>
send to clients (does not implement modifiers yet)

Signed-off-by: Mattias Andrée <maandree@operamail.com>