HAMMER 11/many - initial spike commit.

HAMMER 11/many - initial spike commit.

* Fix a bug in the cluster disk offset calculation.
* Implement the cluster allocator and related header initialization.
* Remove wildcarding from hammer_btree_cmp().
* Move the historical filter out of hammer_btree_cmp() and into its own
  procedure.
* Allow the ending element in a B-Tree range iteration to be inclusive or
  exclusive of the range.
* Add infrastructure for cluster-localizes searches.
* Initial commit of the spike code (still in progress).

This commit brings in most of the infrastructure needed for the spike code.
The spike code is what glues one cluster's B-Tree to another cluster's B-Tree.
At the moment we spike by taking the B-Tree leaf node at the cursor after
a search and moving all of its elements to a new cluster, then replacing
the pointer to that leaf node in the parent B-Tree node with a cluster
reference.

This is non-optimal at the moment.  Several optimizations are possible and
will be eventually be implemented.