Once again about porting LumoSQL to use libmdbx

[erthink]

Basically, I created issue to declare intention to do such porting, so that those interested could find a project or write something in response.

The main motivation is that libmdbx has eliminated or significantly mitigated many of the shortcomings of LMDB, as well as added new features (too much to write about here). Therefore, it suggests that LumoSQL based on libmdbx will be a significantly more advanced product.

The other side of motivation comes down to gaining experience and understanding SQLite's needs and features of an underlying storage engine, in order to then implement ones in libmdbx.

I will be grateful for your advice and suggestions, both here and in LumoSQL/issue#12.