IMAP: sync when requested.
IMAP: call the folder's handler like the mailbox.
IMAP: allow only one sync at a time, divided header fetching.

* CheckMailboxesCommand, and FetchHeadersCommand now inherit from SyncCommand
  which will prevent new CheckMailboxesCommand to be enqueued.
* FetchHeadersCommand now only retrieves up to kMaxFetchEntries headers at
  once. This gets the same TODO about scaling as the limit in
  CheckMailboxesCommand when fetching the flags/UIDs. Since we already read
  all new UIDs at that point, we could easily do better there, though.