lib/index: Implement new indexer

Implement a new message indexer consisting of a single-threaded scanner
and a multi-threaded indexer.

This allows for a number of optimizations as well as background
indexing, though this initial version should be behave similar to the
old indexer.

mu/mu-cmd-find.cc

@@ -28,7 +28,6 @@
 
 #include "mu-msg.h"
 #include "mu-maildir.h"
-#include "mu-index.h"
 #include "mu-query.h"
 #include "mu-msg-iter.h"
 #include "mu-bookmarks.h"