Fix health check time out.

Fix health check time out.

Heal check time out could happen in several places:

1) connect system call
2) select system call
3) read system call

1) was ok but in 2) and 3) it was possible that it could go into
infinite loop in Pgpool-II 3.7 or later. This was due to a mistake
when health check process was modified to be separate process in 3.7,
and back patched to 3.7 and above.

Discussion:
https://www.pgpool.net/pipermail/pgpool-hackers/2019-October/003458.html
https://www.pgpool.net/pipermail/pgpool-hackers/2019-October/003459.html