Dumping file buckets puts the BridgeDB database in an unrecognized state

When we're dumping bridges to files using the --dump-bridges option, BridgeDB changes the distributor from unallocated to pseudo-name in the database. However, when starting BridgeDB normally afterwards, it has no idea to which ring to add this bridge. In particular, BridgeSplitter.insert() exits in line ring.insert(bridge) with a traceback.

This problem is related to #2557 (moved).