Pathological IVORN causes failure to release eventdb lock

[jdswinbank]

If we create an event with the IVORN ivo://, then the event db check will attempt to create a database at ${eventdb_root}, which is a directory. This will fail, and the event will be rejected, which is fine.

However, there is now an entry called "" in the list of databases. When we try to prune the databases, that will get locked, but the prune fails and the database is never unlocked. Our event validation then seizes up.

[jdswinbank]

Every time a new event with the broken IVORN is received, we attempt to acquire the lock on the "" database which hasn't been released. That creates a new thread, which sits their waiting forever. Eventually, we use up our thread limit, and can create no new threads, so nothing except the main loop can run -- no validation, no nothing.