Prevent duplicate symbol placement requests

[kkaefer]

In some situations, we are doing the same symbol placement request multiple times:

• placement configuration changed => triggers request
• rerender trigger update, which triggers a null change, but since the original request didn't complete yet, we are starting another placement request

[mention-bot]

@kkaefer, thanks for your PR! By analyzing the history of the files in this pull request, we identified @jfirebaugh, @incanus and @brunoabinader to be potential reviewers.