Add more LOG and DEBUG messages for slot synchronization.

This provides more information about remote slots during synchronization
which helps in debugging bugs and BF failures due to test case issues. We
might later want to change the LOG message added by this patch to DEBUG1.

Author: Hou Zhijie
Reviewed-by: Amit Kapila, Bertrand Drouvot
Discussion: https://postgr.es/m/514f6f2f-6833-4539-39f1-96cd1e011f23@enterprisedb.com
Discussion: https://postgr.es/m/OS0PR01MB571633C23B2A4CAC5FB0371A944C2@OS0PR01MB5716.jpnprd01.prod.outlook.com

Author: Amit Kapila

src/backend/replication/logical/slotsync.c

@@ -321,6 +321,9 @@ reserve_wal_for_local_slot(XLogRecPtr restart_lsn)
 			oldest_segno = XLogGetOldestSegno(cur_timeline);
 		}
 
+		elog(DEBUG1, "segno: %ld of purposed restart_lsn for the synced slot, oldest_segno: %ld available",
+			 segno, oldest_segno);
+
 		/*
 		 * If all required WAL is still there, great, otherwise retry. The
 		 * slot should prevent further removal of WAL, unless there's a
@@ -361,7 +364,18 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 * current location when recreating the slot in the next cycle. It may
 		 * take more time to create such a slot. Therefore, we keep this slot
 		 * and attempt the synchronization in the next cycle.
+		 *
+		 * XXX should this be changed to elog(DEBUG1) perhaps?
 		 */
+		ereport(LOG,
+				errmsg("could not sync slot information as remote slot precedes local slot:"
+					   " remote slot \"%s\": LSN (%X/%X), catalog xmin (%u) local slot: LSN (%X/%X), catalog xmin (%u)",
+					   remote_slot->name,
+					   LSN_FORMAT_ARGS(remote_slot->restart_lsn),
+					   remote_slot->catalog_xmin,
+					   LSN_FORMAT_ARGS(slot->data.restart_lsn),
+					   slot->data.catalog_xmin));
+
 		return;
 	}