hammer2 - Implement I/O abstraction, fix deadlocks

hammer2 - Implement I/O abstraction, fix deadlocks

* Implement a device buffer I/O abstraction, struct hammer2_io.  This
  abstraction allows multiple chains using smaller allocations to be
  locked concurrently sharing the same (larger) underlying buffer cache
  buffer.

* Does a very good job abstracting device block size selection from
  logical block size selection.

* The new implementation also abstracts asynchronous callbacks quite well.

* Not currently all that SMP friendly.

* Fixes buf/bio deadlocks under heavy loads.