'Searches the database for pairs of (hash, offset) values.
hashes: A sequence of tuples in the format (hash, offset)
-   hash: Part of a sha1 hash, in hexadecimal format
- offset: Offset this hash was created from/at.
Returns a sequence of (sid, offset_difference) tuples.
sid: Song identifier
offset_difference: (offset - database_offset)'

@abc.abstractmethod
def return_matches(self, hashes):

    pass
