feat: option to log flux queries cancelled because of server shutdown

[williamhbaker]

Closes #21286

Results in a log line like this if the server is shutdown with SIGINT while a flux query is in-progress:

2021-12-29T21:34:54.612657Z	info	Cancelling Flux query because of server shutdown	{"log_id": "0Yj1cVWW000", "service": "storage-reads", "query": "import \"http\" http.post(url: \"http://localhost:8080\") from(bucket: \"bucket\") |> range(start: 0) |> yield()"}

[lesam]

See comment