go-imap v2 Backend interface work
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Closes #323.
Speaking of backendutil helpers, I extracted https://github.com/foxcpp/go-imap-sequpdate from my go-imap-maildir experiments. I am not putting it into this PR as I believe it is not polished out enough yet.
Note to self: go-imap-namespace needs updating for the new Interface
Note to self: SendUpdate might need synchronization.
Another note to self: Need to somehow workaround sending of updates in response to CLOSE. This is potentially harmful deviation from RFC 3501.
TODO: Documentation should be cleaned from Updater, etc references.
Just rebased it onto master. If we synchronize dev with master conflicts should be gone.
Closing because this is superseded by go-imap v2.