o Defer costly operations to a pool of threads.
  o Split the internal representation of messages into three distinct documents:
    1) Flags 2) Headers 3) Content.
  o Make use of the Twisted MIME interface.
  o Add deduplication ability to the save operation, for body and attachments.
  o Add IMessageCopier interface to mailbox implementation, so bulk moves
    are costless. Closes: #4654