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.