One of my pending work items for post-realm merge.
The lowest-level import task is no longer asynchronous, as we don't want
it to span multiple threads to allow easier interaction with realm.
Removing the `Task` spec simplifies a heap of usages.
Individual usages should decide whether they want to run the import
asynchronously, by either using an alternative override or spooling up a
thread themselves.
This matches the implementation we have for `BeatmapInfo` and
`BeatmapSetInfo`. All comparisons of `OnlineID` should be done directly
using them (ie. how it's done in `ScoreModelDownloader`).