libtracker-miner: Add watchdog for failed unmounts
libtracker-miner: Add watchdog for failed unmounts

Fixes NB#248873. If, for whatever reason, an unmount operation fails
or doesn't respond in a timely fashion, TrackerStorage will emit
::mount-point-added again so Tracker keeps monitoring any further
change in there.

GNOME/libmediaart,GNOME/libmediaart