Very unstable work-in-progress. btree_node parent() returns btree_node_ptr instead of btree_node* in attempt to increase robustness. m_branch_insert now has responsibility for keeping the child's parent() up-to-date.
Add buffers_in_use().
Add asserts to m_leaf_insert to verify parent and parent_element get updated on new node after split. Fix bugs that these asserts detected. A few minor robustness changes.
Expand use of inspect_leaf_to_root, but not covering complex cases yet. Continue incremental improvements to reporting.