atomically updates

[HuKeping]

In my opinion, the UpdateMany() should either update all the record or nothing. And it do use transaction in the database one:
https://github.com/docker/notary/blob/master/server/storage/database.go#L73-L75

But the memory one seems not:
https://github.com/docker/notary/blob/master/server/storage/memory.go#L63-L69

[endophage]

MemoryStore is only used for testing so this is non-critical. However it would be ideal to have the stores all implement the interface in a consistent manner so definitely something to be corrected.

[riyazdf]

Resolved with #824