Let table AM insertion methods control index insertion

Previously, the executor did index insert unconditionally after calling
table AM interface methods tuple_insert() and multi_insert().  This commit
introduces the new parameter insert_indexes for these two methods.  Setting
'*insert_indexes' to true saves the current logic.  Setting it to false
indicates that table AM cares about index inserts itself and doesn't want the
caller to do that.

Discussion: https://postgr.es/m/CAPpHfdurb9ycV8udYqM%3Do0sPS66PJ4RCBM1g-bBpvzUfogY0EA%40mail.gmail.com
Reviewed-by: Pavel Borisov, Matthias van de Meent, Mark Dilger

src/backend/commands/tablecmds.c

@@ -6360,8 +6360,12 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
 
 			/* Write the tuple out to the new relation */
 			if (newrel)
+			{
+				bool		insertIndexes;
+
 				table_tuple_insert(newrel, insertslot, mycid,
-								   ti_options, bistate);
+								   ti_options, bistate, &insertIndexes);
+			}
 
 			ResetExprContext(econtext);