Reduce relcache access in WAL sender streaming logical changes

get_rel_sync_entry(), which is called each time a change needs to be
logically replicated, is a rather hot code path in the WAL sender
sending logical changes.  This code path was doing a relcache access on
relkind and relpartition for each logical change, but we only need to
know this information when building or re-building the cached
information for a relation.

Some measurements prove that this is noticeable in perf profiles,
particularly when attempting to replicate changes from relations that
are not published as these cause less overhead in the WAL sender,
delaying further the replication of changes for relations that are
published.

Issue introduced in 83fd4532.

Author: Hou Zhijie
Reviewed-by: Kyotaro Horiguchi, Euler Taveira
Discussion: https://postgr.es/m/OS0PR01MB5716E863AA9E591C1F010F7A947D9@OS0PR01MB5716.jpnprd01.prod.outlook.com
Backpatch-through: 13

src/backend/replication/pgoutput/pgoutput.c

@@ -1008,8 +1008,6 @@ static RelationSyncEntry *
 get_rel_sync_entry(PGOutputData *data, Oid relid)
 {
 	RelationSyncEntry *entry;
-	bool		am_partition = get_rel_relispartition(relid);
-	char		relkind = get_rel_relkind(relid);
 	bool		found;
 	MemoryContext oldctx;
 
@@ -1041,6 +1039,8 @@ get_rel_sync_entry(PGOutputData *data, Oid relid)
 		List	   *pubids = GetRelationPublications(relid);
 		ListCell   *lc;
 		Oid			publish_as_relid = relid;
+		bool		am_partition = get_rel_relispartition(relid);
+		char		relkind = get_rel_relkind(relid);
 
 		/* Reload publications if needed before use. */
 		if (!publications_valid)