Log in debug-mode.
  Only sort messages if the selection changes.
  Update debug-log when changing cached count of mails.
  Cache all headers.

  Rather than caching "From:" + "To:" we instead cache all
 header name + value pairs from the message.  This means when
 we have something like this:

    index_format( "[$FLAGS] $DAY/$MONTH/$YEAR $FROM - $SUBJECT" )

  We only open the the message once to read all the headers.
 This is win that is part of #140.