add support for libmdbx (revised and extended descendant of LMDB)

[erthink]

I am the main developer of libmdbx and very interested in your project.
I think I will be able to implement the driver for MDBX and issue a pull-request.
However, I would like to hear your approval of the idea in advance. And also get tips and suggestions.

For more information about libmdbx I suggest you to read Improvements over LMDB section.

Regards.