We're planning to switch from a block-based
exception index to an extent-based index.
This is a tricky thing.  Once this is designed,
but before it's implemented, we probably want
to add a unit test for the btree code,
and use that to help bring up the new index.

This depends on the unit test framework (issue 29).
It probably will require some changes to better separate
the btree code from the rest of ddsnap;
see writeup in
http://zumastor.googlecode.com/svn/trunk/doc/Cleanup_roadmap.txt
The reason to defer writing the unit test until
after the extent-based index is designed is
to help make sure that when we hoist btree
out of ddsnap, we do it in a way that isn't
likely to interfere with the extent rewrite.