hammer2 - Stabilize new I/O infrastructure and work on bulkfree

hammer2 - Stabilize new I/O infrastructure and work on bulkfree

* Stabilize the new asynchronous hammer_io infrastructure.

* Document the new simpler bulkfree scan algorithm.  The new algorithm
  scans meta-data in two passes, rebuilding the freemap in kernel memory.
  It then compares the map against the live media.

  The first pass transitions the freemap from (11 = allocated) to
  (10 = staged for deletion), the second pass transitions (10) either back
  to (11) if the block is found to not actually be freeable or to (00) if
  the block winds up being actually freeable.

  The two pass mechanism deals with any races and allows the scan to occur
  essentially unlocked,

* Start work on the bulkfree scan algorithm.