Added yet another thread, the ConfigMonitor. Since that thread needs to run
code to reload cached config values from the git-annex branch when files
there change, writing it also let me review where config files are cached,
and I found that every single config file in the git-annex branch does
get cached, with the exception of the uuid.log. So, added a cache for that,
and now I'm more sanguine about yesterday's removal of the lower-level
cache, because the only thing not being cached is location log information.

The ConfigMonitor thread seems to work, though I have not tested it
extensively. The assistant should notice and apply config changes
made locally, as well as any config changes pushed in from remotes.
So, for example, if you add a S3 repo in the webapp, and are paired with
another computer, that one's webapp will shortly include the new repo in
its list. And all the preferred content, groups, etc settings will
propigate over and be used as well.

Well ... almost. Seems nothing causes git-annex branch changes to be
pushed, until there's some file change to sync out.