hammer2 - Major hammer2_chain_*() API cleanup

hammer2 - Major hammer2_chain_*() API cleanup

* Do a major rewrite of the hammer2_chain_*() API functions, primarily
  dealing with whether the caller wants data instantiated for a chain
  element or not.

  Also revamp the locking calls.  Integrate the ref count into
  hammer2_chain_lock() to simplify the operation.

* Add infrastructure to allow a minimum physical I/O size to be specified
  independently from the allocation size.  Not finished yet and currently
  disabled.

* Optimize I/O a little, but this still needs a ton of work.
  Set B_CLUSTEROK for logical buffers.

* Add some statistics gathering.

* Remove several duplicative functions.