tracker-store: Automatically shutdown on inactivity

If the right conditions apply, tracker-store will shut down after 30s of inactivity (no clients doing updates/selects). Bringing it back again is relatively cheap, so let's see how this flies.

For the cases it won't, tracker-store has a --disable-shutdown switch (also useful for testing from terminal), also running on other buses than the session one will disable it, since both shutting down and later restart pose questions and risks.

In theory, this will make tracker-store disappear 99% of the time, since database updates are sparse. There's also the possibility of clients running with TRACKER_SPARQL_BACKEND=bus or resorting to bus connection (eg. flatpak apps), that will make selects go through tracker-store.