Lox bridge_table HashMaps should find the next available key

This implements and adds extensive testing for a find_next_available_key function for the Lox bridgetable, addressing #23 (closed). Some important background info: open_invitations are added to the trustup migration table when added to Lox. Each one points to their superset bucket that can be accessed after 30 days (in our current design). All credentials expire after 511 days, so that is the last time a user who is eligible to level up will be able to. That is a very long time though so we could also shorten this. Likewise, blocked bridges are immediately added to the migration table but users may want to continue migrating long after the actual blockage is confirmed (possibly up to 511 days). For this reason, I decided that the ids of these buckets could safely be recycled after this 511 day period.