mod_s2s: Close connection on smacks timeout
This merges the mod_s2s_smacks_timeout behavior from prosody-modules
This event is fired by mod_smacks when the connection has not responded
to an ack-request for a period of time defaulting to 30 seconds,
indicating that the connection has become stuck or non-responsive.
Closing it prevents routing further messages via this connection and
frees resources. A stuck connection may otherwise remain until for a
time determined by the OS TCP subsystem, which can be quite long.