set danger in mds-base's receive_danger

Signed-off-by: Mattias Andrée <maandree@operamail.com>
do not resend danger to other threads

Signed-off-by: Mattias Andrée <maandree@operamail.com>
pack client_list on danger

Signed-off-by: Mattias Andrée <maandree@operamail.com>
mds-registry: take care of danger

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