I have a test for this, and this is actually the reason why currently I need the Vector actually: `persistAsyncInvocations: mutable.HashMap[Any, Vector[AsyncPersistInvocation]]`, so for `persistAsync(a); persistAsync(a)` I'll get two handlers. So it's currently like a `MultiMap`, but MultiMap does not do ordering (value is a Set there), which is why a manual impl. here

// I'm still thinking if I could get away with just a Vector here - did a failed attempt previously.