Store Lox bridge table and spent credential tables in a disk-backed database

As part of #6 (closed), the Lox distributor, particularly the context manager, needs to keep a record of the Lox bridge table and the spent credential ids for each of the Lox credential types on some kind of disk-backed database that can be read in at start up in the case of a catastrophic failure and potentially be used to restore from a previous state. This might be needed if, for example, all of the bridges in an area got blocked due to a censor's new technique that could identify all of a certain type of bridge by protocol (by no fault of Lox users). In such a case, many users will be locked out of their trust buckets and some users may have already attempted to migrate to new bridges and had their credentials updated with a penalty. If the bridges blocked by protocol could be replaced with a new type of bridge, it might be desirable to roll back the spent credential id tables to an earlier state and allow users to replay credentials used after a certain date.