Add btree_node owner() function. Add BOOST_ASSERT to ensure what we think is the root is at the right level.
Change header levels() to return  because it is disconcerting to write levels() to a stream like cout and have it be displayed as an unsigned char, which is treated as '' for typical values of levels().
Add cache_size_test and options processing
Clarify buffer stats message. Expose buffer use counts. Add buffer_manager clear_cache function and use it to prevent gratuitous cache entry caused by writing the initial header record on a new or truncated file.