tracker-miner-fs `--domain-ontology` flag has no effect on the tracker-extract watchdog

tracker-miner-fs has a watchdog which watches org.freedesktop.Tracker1.Miner.Extract on D-Bus and restarts it when it crashes.

When running in a domain other than the default, this is the wrong thing to do -- it should be watching the domain-specific tracker-extract process and restarting that.

We could fix this by calling the tracker_sparql_connection_get_domain() method and using the domain passed by that.

That seems a little ugly though; I might instead modify libtracker-miner so that the TrackerMiner base class has a 'domain' property.