hammer2 - Change freemap segment delineation, other stuff

hammer2 - Change freemap segment delineation, other stuff

* Change the freemap delineation to encode the BREF type as
  well as the radix, in order to avoid mixing data and meta-data
  types together in the same device buffer.

* Reorganize the freemap to remove one indirection level.  Reserve
  two bits per block instead of one.  Change bitmap granularity to 16KB
  but leave allocation granularity at 1KB.

* Fix hammer2_devblksize() to match hammer2_devblkradix().

* Add hammer2_chain_iterate(), a chain iteration with a
  callback.

* Fix issue when hammer2_chain_get() is called with HAMMER2_LOOKUP_ALWAYS.

* Start working on proper statistics rollups during flush (not complete).