*** Reason for rollback ***
We figured out a simpler solution affecting fewer files - we
don't actually need the new remotestore API.
*** Original change description ***
Some stores require changing the UID of a message when it is
copied to a new folder (I'm looking at you, EAS). Add a callback
to Folder.copyMessages() which allows the store to report back
such changes. Then, add a new api to record the new values:
Folder.updateMessages().
For now, the two APIs are linked by a common callsite in
MessagingController, so the existing stores can use a minimal
implementation - if they don't call the callback, nobody will
call the update.
BUG=1807499
Automated import of CL 147708
b633efa2b7