Use CompletableFutures for async update checking

[TBlueF]

This PR removes the necessity of calling check() manually before logging the update-messages.
It also moves the async logic to a CompletableFuture to make sure that the actual check-call is only done once even if we call getLatestVersion() multiple times in quick succession. :)

[TBlueF]

Oh, i also removed the technicjelle.updatechecker.noasync option again, since it shouldn't be necessary anymore .. tell me if you still want to keep it ^^

[TechnicJelle]

Do you think the readme also needs to be updated with mentions of these new additions?