Allow selecting Acorn DB backend by system property
In #976 (closed) RocksDB was added as a DB backend option but the backend can still only be enabled by changing the code:
-Dorg.simantics.db.backend=default|fs|rocksdb
In #976 (closed) RocksDB was added as a DB backend option but the backend can still only be enabled by changing the code:
-Dorg.simantics.db.backend=default|fs|rocksdb