From spec: https://jmap.io/spec-core.html#set (section 5.3)

oldState: The state string that would have been returned by Foo/get before making the requested changes, or null if the server doesn’t know what the previous state string was.
newState: The state string that will now be returned by Foo/get.

How

• When a Email/set request is received, we need to fetch the current state of the Email objects. This should be returned as `oldState` property in the response.
• After all the changes in the Mailbox/set request have been applied successfully, we should create a new state, store it in the EmailChangeRepository and return it with the response as `newState` property.
• If all the methodCalls in the request end up failing then no new state should be generated.

  DoD

Integration tests to show that the Mailbox/set method can return oldState & newState property.