An ObjStore::Index optimized for insert performance.

- Representation is a link-list of OSPVptr arrays.

- Caches keys for the first record in a block for better random seek
performance.

- Optimistic indexing constraints; No read-only protection.

The design may seem strange but the performance implications of
persistent memory are also strange.  Whether this is an optimal
solution is still unknown.

I'm beginning to think I'm foolish for avoiding templates.  OTOH...