Medium: pgsql: change stop_escalate parameter as usable value

stop_escalate can be now specified as the actual number of seconds
to wait for the stop completion before escalating to force stop.

It should no affects for users who have not specified stop_escalate.
Only users who have been using this parameter should add +60 when upgrading
as a substitute of waiting in pg_ctl stop.

This change should be along with the commit in PR #795.