Refactor the internal GIN B-tree interface for forming a downlink.

This creates a new gin-btree callback function for creating a downlink for
a page. Previously, ginxlog.c duplicated the logic used during normal
operation.

src/backend/access/gin/ginbtree.c

@@ -452,6 +452,7 @@ ginInsertValue(GinBtree btree, GinBtreeStack *stack, GinStatsData *buildStats)
 			}
 		}
 
+		btree->prepareDownlink(btree, stack->buffer);
 		btree->isDelete = FALSE;
 
 		/* search parent to lock */