We got many reports where the 'lazy check' didn't work too well for
people... so make it a bit less lazy, so it'll just work for more
people.
In practice, never skip _directories_ unless they're leaf directories;
this avoids the mtime-does-not-bubble-up problem.
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.
