This fix stores as multi-line headers that are repeated, and that were
being discarded when storing them in a regular dict.

It had been removed during the last refactor.

I also store headers now as a case-insensitive dict, which solves other
problems with the implementation of the twisted imap.

Releases: 0.4.0

bug: delete_msg was still passing the mbox_name instead of the mbox_uuid
as it should.

The previous implementation is naive, since it imposes a burden when
renaming mailboxes. We're using uuids in the local uid tables instead,
which is more cryptic but way more efficient.

* receive mbox uuid instead of name
* use mailbox uuid in identifiers