HAMMER 27/many: Major surgery - change allocation model

HAMMER 27/many: Major surgery - change allocation model

After getting stuck on the recovery code and highly unoptimal write
performance issues, remove the super-cluster/cluster and radix tree bitmap
infrastructure and replace it with a circular FIFO.

* Nothing is localized yet with this major surgery commit, which means
  radix nodes, hammer records, file data, and undo fifo elements are
  all being written to a single fifo.  These elements will soon get their
  own abstracted fifos (and in particular, the undo elements will get a
  fixed-sized circular fifo and be considered temporary data).

* No sequence numbers or transaction spaces are generated yet.

* Create a 'hammer_off_t' type (64 bits).  This type reserves 4 bits for
  a zone.  Zones which encode volume numbers reserve another 8 bits,
  giving us a 52 bit byte offset able to represent up to 4096 TB per
  volume.  Zones which do not encode volume numbers have 60 bits available
  for an abstracted offset, resulting in a maximum filesystem size of
  2^60 bytes (1 MTB).  Up to 15 zones can be encoded.

  As of this commit only 2 zones are implemented to wrap up existing
  functionality.

* Adjust the B-Tree to use full 64 bit hammer offsets.  Have one global B-Tree
  for the entire filesystem.  The tree is no longer per-cluster.

* Scrap the recovery and spike code.  Scrap the cluster and super-cluster
  code.  Scrap those portions of the B-Tree code that dealt with spikes.
  Scrap those portions of the IO subsystem that dealt with marking a
  cluster open or closed.

* Expand the hammer_modify_*() functions to include a data range and add
  UNDO record generation.  Do not implement buffer ordering dependancies
  yet (ordering issues are going change radically with the FIFO model).