HAMER VFS - Cache additional b-tree nodes in directory.

HAMER VFS - Cache additional b-tree nodes in directory.

* Cache inode record cursors in the directory at dip->cache[2] to reduce
  cpu overhead when iterating a directory.

* Cache file object record cursors in the file's governing directory at
  dip->cache[3] after reading the last data record in a file, to reduce
  cpu overhead when chain-reading files.

* Also add vfs.hammer.stats_btree_root_iterations, a counter which
  is incremented whenever a B-Tree search is initiated from the root
  node.