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/include/access/gin_private.h

@@ -471,4 +471,6 @@ ginCompareItemPointers(ItemPointer a, ItemPointer b)
 		return -1;
 }
 
+extern int ginTraverseLock(Buffer buffer, bool searchMode);
+
 #endif   /* GIN_PRIVATE_H */