HAMMER part 2/many.

HAMMER part 2/many.

* Implement most of the I/O infrastructure and internal HAMMER tracking
  structures for volumes, super-clusters, clusters, and buffers.

* Flesh out the B-Tree code and add an iterator.

* Implement a good chunk of the vnops, but no modifying operations yet.

* Implement passive filesystem buffer tracking which allows a struct buf
  to remain associated with internal HAMMER data structures and also
  provides a reverse path whereby the filesystem buffer cache drives
  garbage collection of internal HAMMER data structures.  Use the augmented
  bio_ops to facilitate this.

* Skeleton for transactions, spikes, and object allocation & management.