Request package for lmdb-rkv

[jw3]

The rawhide package requirements have been reduced significantly with some recent build changes, it looks like we are down to lmdb-rkv and its dependencies.

https://github.com/mozilla/lmdb-rs

Note that there is an existing package for lmdb

https://github.com/danburkert/lmdb-rs

but that project is dated, and lmdb-rkv is a Mozilla fork of it that added 178 commits.

Attempted a quick switch to lmdb to test, but the api appears to have changed, did not dig in.

[jw3]

The lmdb crate may work after all

ref #715

[jw3]

Fixed in #730