Fix a race condition in the AudioLoader

[schnapster]

There are conflicts with the v3 branch. Not sure how we would update both branches with the fix (I can resolve them just not sure whether we merge this fix into both, or merge dev into v3 after this PR is merged)?

[schnapster]

I totally forgot to give some context:

The AudioLoader can lock up, when lavaplayer's loadItem returns (and therefore notifies) a result before the loadSync method has reached the line where it waits. The thread will then wait forever there.

Here's a stack of a waiting thread: https://wastebin.party/detatipocu.css
The issue can be easily reproduced after a few curls to the loadtracks endpoint by using identifiers/loaders that return very fast, like looking up a local audio file.

[freyacodes]

LGTM

[schnapster]

v3 merge done in 0adc5c8