pgsql: Refresh apply delay on reload of recovery_min_apply_delay at rec

Refresh apply delay on reload of recovery_min_apply_delay at recovery

This commit ensures that the wait interval in the replay delay loop
waiting for an amount of time defined by recovery_min_apply_delay is
correctly handled on reload, recalculating the delay if this GUC value
is updated, based on the timestamp of the commit record being replayed.

The previous behavior would be problematic for example with replay
still waiting even if the delay got reduced or just cancelled.  If the
apply delay was increased to a larger value, the wait would have just
respected the old value set, finishing earlier.

Author: Soumyadeep Chakraborty, Ashwin Agrawal
Reviewed-by: Kyotaro Horiguchi, Michael Paquier
Discussion: https://postgr.es/m/CAE-ML+93zfr-HLN8OuxF0BjpWJ17O5dv1eMvSE5jsj9jpnAXZA@mail.gmail.com
Backpatch-through: 9.6

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/942416f4bdb056c2b12a97ef7fb2d21b565df3cb

Modified Files
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src/backend/access/transam/xlog.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)