HAMMER Utility: Change B-Tree, inodes, and dir reblocking params, add rebalance

HAMMER Utility: Change B-Tree, inodes, and dir reblocking params, add rebalance

* The normal cleanup operations now reblock all B-Tree, inode, and directory
  elements in the normal daily reblock mode instead of only the ones in
  fragmented big-blocks.  Bulk data is handled by the 30-day recopy mode.

* Add a new directive 'rebalance' (a future VFS ioctl).  This directive will
  tell the HAMMER VFS to rebalance the B-Tree.  HAMMER B-Trees are always
  balanced by depth but degenerate cases with minimal elements in a node
  can easily build up.  The new directive will rebalance the elements in
  each B-Tree node.

* The hammer cleanup directive was not reblocking directories.  Now it does.

It usually does not take very long to reblock the B-Tree nodes, inodes, or
directory elements.  Reblocking these unconditionally, instead of just
reblocking fragmented allocation areas, keeps the B-Tree in a more optimal
layout, though there is still a lack of correlation between inode numbers
and directory scan order.