Speed up failover when all of backends are down.

Speed up failover when all of backends are down.

Pgpool-II tries to find primary node till search_primary_node_timeout
expires even if all of the backend are in down status. This is not
only a waste of time but makes Pgpool-II looked like hanged because
while searching primary node failover process is suspended and all of
the Pgpool-II child process are in defunct state, thus there's no
process which accepts connection requests from clients. Since the
default value of searching primary is 300 seconds, typically this
keeps on for 300 seconds. This is not comfortable for users.

So immediately give up finding primary node regardless
search_primary_node_timeout and promptly finish the failover process
if all of the backend are in down status.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2019-May/003321.html