HAMMER 3/many - more core infrastructure.

HAMMER 3/many - more core infrastructure.

* Add an in-memory B-Tree node abstraction
* Add an in-memory record abstraction.
* Put the B-Tree cursor code in its own source file.
* Fill in more of the VOP code.

* Do a major clean-up of all in-memory structures and some on-disk
  structures.  All the major in-memory structures now use similarly
  named functions.

* Move inter-cluster link from a B-Tree leaf node to a B-Tree internal
  node, giving us a left and right boundary to play with.  This simplifies
  the algorithms by quite a bit.
* Allow the B-Tree to be unbalanced by moving the sub-type from the B-Tree
  node header to the B-Tree element structure.

* Revamp the I/O infrastructure, in particular allow B-Tree nodes to be
  held passively.
* Implement a flexible B-Tree node cache.  References into the B-Tree can
  be cached by inodes.  If the related buffer is flushed by the system, the
  related cache pointers will be cleared.