`deleteAnnotation` RPC event can be triggered before `loadAnnotations`.
This is a race condition in which annotations are asked to be deleted in
the `sidebar` frame before the `guest` frame finishes to anchor the
annotations.

B
This problem is avoided by maintaining one additional set which tracks
the annotations that should be in the page. This can be updated 1) when
an annotation is received, just before it is anchored and 2) when an
annotation is detached. After anchoring completes, the logic can check
if the annotation should still be present in the page and skip saving
the anchor if not.

Solution from here:
https://github.com/hypothesis/client/pull/4007#discussion_r763971360