voeventdb-to-voeventdb synchronisation functionality

[timstaley]

One the reasons for building voeventdb is to allow 'catch-up', i.e. recovery of voevents in case of a network or systems outage. You can already query voeventdb for recent events, but it would seem sensible to build a dedicated voeventdb-to-voeventdb 'sync' feature that ensures a full synchronisation of the databases (as judged by e.g. list of all IVORNs for VOEvents stored).

I have some very rough ideas about how one might do this efficiently (hashing IVORNs and comparing corpus-hashes, then bissecting to some granularity) but it needs thought and might be tricky to get right. Probably an inefficient but foolproof solution would be a good interim measure (even if feasible over local network only?).