NewDBReader returns a new *DBReader.

NewIterator returns a new *badger.Iterator.

ErrScanBreak is returned by ScanFunc to break the scan loop.

DBReader reads data from DB, for read-only requests, the locks must already be checked before DBReader is created.

ScanProcessor process the key/value pair.

BatchGetFunc defines a batch get function.

ScanFunc accepts key and value, should not keep reference to them.