check_hot_standby_delay: compare database system identifier

[jw1u1]

The system identifiers of master and standby should be compared to ensure that replication and monitoring are working properly.
Alternatively, don't set a negative delta to zero but warn about it:

# Make sure it's always positive or zero
$rec_delta = 0 if defined $rec_delta and $rec_delta < 0;
$rep_delta = 0 if defined $rep_delta and $rep_delta < 0;