Reduce page locking in GIN vacuum

GIN vacuum during cleaning posting tree can lock this whole tree for a long
time with by holding LockBufferForCleanup() on root. Patch changes it with
two ways: first, cleanup lock will be taken only if there is an empty page
(which should be deleted) and, second, it tries to lock only subtree, not the
whole posting tree.

Author: Andrey Borodin with minor editorization by me
Reviewed-by: Jeff Davis, me

https://commitfest.postgresql.org/13/896/

src/backend/access/gin/ginbtree.c

@@ -31,7 +31,7 @@ static void ginFinishSplit(GinBtree btree, GinBtreeStack *stack,
 /*
  * Lock buffer by needed method for search.
  */
-static int
+int
 ginTraverseLock(Buffer buffer, bool searchMode)
 {
 	Page		page;