This allows an LMDB database to be used as the blockchain to export.
Adjust SOURCE_DB in src/blockchain_converter/blockchain_export.h
depending on needs. Defaults to DB_MEMORY.
DB_MEMORY is a sensible default for users migrating to LMDB, as it
allows the exporter to use the in-memory blockchain while the other
binaries work with LMDB, without recompiling anything.