Check hashes against the filestore.

    Blobs use the hash in the schema:
    aff4:/files/hash/generic/sha256/[sha256hash]

    Args:
      hashes: A list of Hash objects to check.

    Yields:
      Tuples of (RDFURN, hash object) that exist in the store.

def CheckHashes(self, hashes):
    """Check hashes against the filestore.

    Blobs use the hash in the schema:
    aff4:/files/hash/generic/sha256/[sha256hash]

    Args:
      hashes: A list of Hash objects to check.

    Yields:
      Tuples of (RDFURN, hash object) that exist in the store.
    """
    hash_map = {}
    for hsh in hashes:
      if hsh.HasField("sha256"):
        # The canonical name of the file is where we store the file hash.
        hash_map[aff4.ROOT_URN.Add("files/hash/generic/sha256").Add(
            str(hsh.sha256))] = hsh

    for metadata in aff4.FACTORY.Stat(list(hash_map)):
      yield metadata["urn"], hash_map[metadata["urn"]]