path: Add find_uris version of find_files
scanner: Write tag cache to tmp file.

Tag cache is now output to a tmp file residing in the same folder as the real
one. Once generated the tmpfile is moved over the original file for an atomic
updated.
core: Add library updater.

Idea behind this class is to have a clear sub-set of our library API that is
only used for updating libraries. This makes it very clear what methods are
needed for just libaries and additionally updatable libraries.

Next couple of commits will build on this idea taking us closer to plugable
local libraries.
local: Add library update provider for tag caches.

This adds the basic interface implemenations for supporting tag caches using
the new update APIs.
scanner: Convert to using library updater.

This version of the scanner switches to using the updater API, in other words
we are now fairly close to being able to plug in alternate libraries for local
files and populate them with mopidy-scan.
scanner: Fixing review comments.
Merge pull request #454 from adamcik/feature/library-updater

Add library update providers to backends