Only include xapian.h in one place, so we can have consistent options.
With that in place, we can enable C++ move semantics.
We don't do anything with that yet, but we check in the meson.build file
to see if we have the required xapian version.
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.
