These modes match those optionally provided as part of the daemon's
--db-type argument.
Argument after the # is interpreted as a composite mode if there's only
one (no comma separated arguments).
Sample usage:
blockchain_import --database lmdb#fastest
blockchain_import --database berkeley#fastest
Multiple specific DB flags are still supported, e.g.
blockchain_import --database lmdb#nosync,nordahead
blockchain_import --database berkeley#txn_nosync