Fix slow/delayed loading on startup

This MR serializes Grilo art lookups, which works wonders for our startup since the async work was done.

Apparently the Grilo art lookup is heavily taxing the system, doing it in parallel basically grinds down any other progress to a halt. In my limited investigation it floods Tracker with smaller queries, Tracker only handles a limited pool at a time and it leaves no room for any other more relevant queries. Also all Grilo priorities are lower (DEFAULT_IDLE or less), while Tracker priorities are higher. So my thinking is that the flood of Tracker queries get handled before anything Grilo/Music related is getting handled again, while the system is overloaded.