Resume conflict-relevant data retention automatically.

Resume conflict-relevant data retention automatically.

This commit resumes automatic retention of conflict-relevant data for a
subscription. Previously, retention would stop if the apply process failed
to advance its xmin (oldest_nonremovable_xid) within the configured
max_retention_duration and user needs to manually re-enable
retain_dead_tuples option. With this change, retention will resume
automatically once the apply worker catches up and begins advancing its
xmin (oldest_nonremovable_xid) within the configured threshold.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/OS0PR01MB5716BE80DAEB0EE2A6A5D1F5949D2@OS0PR01MB5716.jpnprd01.prod.outlook.com