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.