Automate key rotation process for Lox keys

Further to !183 (merged), there needs to be an automated key rotation process for each of the keys for the lox-distributor. This should operate such that when some threshold of requests have been processed, the keys are rotated so that the dupfilters don't get too large (due to recording spent lox id which are scalars). All keys should be rotated eventually but the keys for lox credentials, trusted invitation credentials and open invitation credentials are the only ones where the old spent keys must also be kept for some time so that unspent old credentials can be updated to the new keys.