'Remove expired nonces from the store.
Discards any nonce from storage that is old enough that its
timestamp would not pass L{useNonce}.
This method is not called in the normal operation of the
library.  It provides a way for store admins to keep
their storage from filling up with expired data.
@return: the number of nonces expired.
@returntype: int'

def cleanupNonces(self):

    raise NotImplementedError
