When an annotation is created or updated the sidebar briefly transitions into the
`waitingToAnchor` state and hides the entire thread list, losing the user's scroll
position as a result.
Instead of hiding the entire thread, filter out the annotations which are waiting to
anchor.

https://trello.com/c/hQRpsUPj/444-avoid-hiding-thread-list-while-new-annotations-received-over-ws-are-being-anchored