We need a mechanism that cleans up old issues in the running state as it is loaded into Zino.

The rationale for this is that sometimes, bugs will cause the saved state to be inconsistent or buggy, and a Zino release that fixes the bugs may also need to clean up old inconsistencies that have been left behind in the state file.

A concrete example is the recent issue described in #481. There is actually more to this: Old state files may contain duplicate events, causing this problem to persist even after upgrading Zino. An initial task in a cleanup routine would be to decide what to do with duplicate events. Should they be somehow merged, or should one of them be closed, etc.