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/matview.c

@@ -476,6 +476,7 @@ static bool
 transientrel_receive(TupleTableSlot *slot, DestReceiver *self)
 {
 	DR_transientrel *myState = (DR_transientrel *) self;
+	bool		insertIndexes;
 
 	/*
 	 * Note that the input slot might not be of the type of the target
@@ -490,7 +491,8 @@ transientrel_receive(TupleTableSlot *slot, DestReceiver *self)
 					   slot,
 					   myState->output_cid,
 					   myState->ti_options,
-					   myState->bistate);
+					   myState->bistate,
+					   &insertIndexes);
 
 	/* We know this is a newly created relation, so there are no indexes */