DVO SkyTable corruption for mis-matched groups

[eugene]

Roger Lin has found a problem working with a DVO database where different group IDs apparently interacted with the DVO updates to the SkyTable.fits to re-write the SkyTable with an incorrect set of values.

I think the issue here is that the code is checking for permission to lock the skytable, but due to the group IDs, there is an inconsistency with the request for lock and the permission check. somehow, DVO decides that it needs to update the SkyTable as a result.