HAMMER 20B/many: New spike topology, simplify the B-Tree code.

HAMMER 20B/many: New spike topology, simplify the B-Tree code.

* Specify a spike as two B-Tree leaf elements instead of one B-Tree
  internal element.  This simplifies boundary corrections when traversing
  through internal nodes.

* Remove subtree_count, which means we don't have to recurse through the
  parent nodes to update it any more.

* Simplify the recursive deletion case.  Neither Leaf or internal nodes
  can be empty.  If unable to remove a node due to a deadlock, simply zero
  out the subtree_offset in the parent (internal) node and deal with it
  later.

* Add some Debugger() shims for deletion cases not yet handled.