Collect statistics about conflicts in logical replication.

This commit adds columns in view pg_stat_subscription_stats to show the
number of times a particular conflict type has occurred during the
application of logical replication changes. The following columns are
added:

confl_insert_exists:
        Number of times a row insertion violated a NOT DEFERRABLE unique
        constraint.
confl_update_origin_differs:
        Number of times an update was performed on a row that was
        previously modified by another origin.
confl_update_exists:
        Number of times that the updated value of a row violates a
        NOT DEFERRABLE unique constraint.
confl_update_missing:
        Number of times that the tuple to be updated is missing.
confl_delete_origin_differs:
        Number of times a delete was performed on a row that was
        previously modified by another origin.
confl_delete_missing:
        Number of times that the tuple to be deleted is missing.

The update_origin_differs and delete_origin_differs conflicts can be
detected only when track_commit_timestamp is enabled.

Author: Hou Zhijie
Reviewed-by: Shveta Malik, Peter Smith, Anit Kapila
Discussion: https://postgr.es/m/OS0PR01MB57160A07BD575773045FC214948F2@OS0PR01MB5716.jpnprd01.prod.outlook.com

src/include/replication/conflict.h

@@ -14,6 +14,12 @@
 
 /*
  * Conflict types that could occur while applying remote changes.
+ *
+ * This enum is used in statistics collection (see
+ * PgStat_StatSubEntry::conflict_count and
+ * PgStat_BackendSubEntry::conflict_count) as well, therefore, when adding new
+ * values or reordering existing ones, ensure to review and potentially adjust
+ * the corresponding statistics collection codes.
  */
 typedef enum
 {
@@ -42,6 +48,8 @@ typedef enum
 	 */
 } ConflictType;
 
+#define CONFLICT_NUM_TYPES (CT_DELETE_MISSING + 1)
+
 extern bool GetTupleTransactionInfo(TupleTableSlot *localslot,
 									TransactionId *xmin,
 									RepOriginId *localorigin,