Internet mail messages (RFC 5322, MIME, RFC 2047/2231 and RFC 6532 UTF-8 headers,
RFC 6858 downgrading) and the mailbox store shared by the Parley SMTP, IMAP and POP3
servers. Messages and mailboxes live in a parley-files FileSource, so messages can be
larger than memory. Split out of us.bringardner:bjl_email.
