Re-introduce in-flight request indication

When people submit heavy requests to ES it may take time for it to respond. We used to have a crude indication of the fact that we had an in flight request but we lost it in the re-design. Adding an issue to remember to introduce an alternative.

No file, test, or entry point is named. First locate the request submission flow and investigate how the pre-redesign in-flight indication worked; clarify the intended alternative and define completion as a visible indication while heavy Elasticsearch requests are pending.

Written by the indexing model from the issue text.