Update all cc code using .clang-format; please do so as well for future PRs
etc.; emacs has a handy 'clang-format' mode to make this automatic.
For comparing old changes with git blame, we can disregard this one using
--ignore-rev
(see https://www.moxio.com/blog/43/ignoring-bulk-change-commits-with-git-blame )
Rewrite the query machinery in c++:
- use an MSet decorator instead of the mu-msg-iter stuff
- use mu-query-decider to mark duplicates/unreadable/related messages
- use mu-query-threader to replace the older container/thread code
Algorithm did not substantially change, but the implementation details
did.