Keep track of the mount-point ordering in a state variable, rather
than a global. This path is tested by existing unit tests.
Note a prior change inserted the MountNode objects directly into a
list in self.state, which makes sorting quite easy as it can just
implement __lt__. Unfortunately we still json dump the state, and
thus we can't have aribtrary objects in it (future work may be to
check keys inserted into the status object...). So we have to do a
bit of wrangling with tuple lists and comparision functions here, but
it's not too bad.
Change-Id: I0c51e0c53c4efdb7a65ab0efe09a6780cb1affa8
09dee46579