core: update Concurrency bit to allow for snapshot view rather than requiring locking

[brong]

From Chris Newman:

3.6 Concurrency

For potentially long-running operations like search and copy/move-messages, the creates an inappropriate and unnecessary implementation burden on the server. In general, the goal is that the server should present a consistent view of data to the client, even if it's slightly out-of-date by the time the method completes. It's fine to state this and even say the ability of a server to present a consistent view to clients is a quality-of-implementation issue for the server. But don't force the presence of a hard locking mechanism into the server implementation unless it's absolutely necessary as that harms server scalability.

Normative restrictions on concurrency should be specified for those methods that need them and be minimally restrictive. For example, a multi-message move operation should either move all messages or none of them, and the messages should retain their relative ordering in the destination folder. But additional restrictions are not needed to present a good client experience.