Implementation of the new sync service that interleaves syncing jobs for
all active users.
It also includes improvements to the message downloader. The download
will now auto rate limit the parallel workers based on the server
responses.
Additionally each of the stages is now tested in isolation to ensure the
behavior matches the expectations.
Finally, this patch does not replace the existing IMAP sync. A follow up
patch is necessary to integrate the IMAP bits into the interfaces
required by these changes.
