Change the name of the "Message" object to remove confusing messageId naming

[brong]

Raised by Dave during IETF99. The name 'messageId' is very confusingly similar to the RFC5322 'Message-Id' header. This will be an ongoing cause of confusion, and should be nipped in the bud.

From the discussion, calling it something else is bad because it breaks the correspondence between object name and {object}Id which is consistent through all the rest of JMAP. Better to rename Message. We need to choose a name.

[neilj]

Looking through the thesaurus, the best alternative is clearly "Email". So we rename the "Message" object to be the "Email" object, which therefore may be referenced by emailId – I think the name is still clear, and removes the ambiguity.

[jeffpc]

"mail" is another option