Cache the RMG Database in a pickle and a hash, to speed up reloading it.

We hash a bunch of metadata to try to be sure that the cached database is the 
same as what would be loaded if you loaded it from scratch. Hopefully I have
included everything that matters.
Use scoop.shared module to share database cache location and hash.

We pass around the location and hash of the database cache file so 
that each worker can load it and check it has the right version.

This removes the need for the "RMG_DB_FILE" environment variable.