This commit changes the startup process in the standby server so that
it handles the interrupt signals after waiting for wal_retrieve_retry_interval
on the latch and resetting it, before entering another wait on the latch.
This change causes the standby server to promptly handle interrupt signals.

Otherwise, previously, there was the case where the standby needs to
wait extra five seconds to shutdown when the shutdown request arrived
while the startup process was waiting for wal_retrieve_retry_interval
on the latch.

Author: Fujii Masao, but implementation idea is from Soumyadeep Chakraborty
Reviewed-by: Soumyadeep Chakraborty
Discussion: https://postgr.es/m/9d7e6ab0-8a53-ddb9-63cd-289bcb25fe0e@oss.nttdata.com