Resolve inconsistencies in timestamp annotation

[jsstevenson]

A few related inconsistencies or things to reconsider or revisit

• Timestamps get added for variants submitted directly to the /variation, /vrs_variation etc endpoints, but not to anything registered via VCF
• Timestamps can only be added once for a variant. I'm not sure this is desirable behavior? Maybe it is?
• Timestamps get added no matter what (you can't disable this)