Solution to be able to upgrade existing instance when bdb will be no more available

[progier389]

Describe the problem
Berkeley Database is deprecated so libdb may not be available in some future, and we need a solution
to allow people to migrate their bdb database to lmdb once libdb is no more available.

Describe the solution you'd like
librpm had the same problem and solved it by having a set of functions able to read records from bdb databases.
The idea is to reuse the functions and glue some code to mimic the bdb API for the read operations to support a minimum set of operations:

• dsctl instance db2ldif backendname ldifpath
• dbscan -L dbpath
• dbscan --export exported_changelog_path -f changelog_path
• dsctl instance bdb2mdb

Describe alternatives you've considered
Forcing people to migrate the db before upgrading (with the risk that someone get somehow stuck with unreadable database)