HAMMER 28/many: Implement zoned blockmap

HAMMER 28/many: Implement zoned blockmap

* Implement a zoned blockmap.  Separate B-Tree nodes, records, small blocks
  of data, and large blocks of data into their own zones.  Use 8MB large
  blocks, 32-byte blockmap entry structures, and two layers to support
  59 bits (512 petabytes).

* Create a temporary freeblock allocator so the blockmap can be tested.
  It just allocates sequentially and asserts when it hits the end of the
  volume.   This will be replaced with a real freeblock allocator soon.

* Clean up some of the mess I created from the temporary fifo mechanism
  that had been put in-place to test the major rewiring in 27.

* Adjust newfs_hammer.  The 'hammer' utility has not yet been adjusted
  (it can't decode blockmaps yet but will soon).