Core: add ignore_idle_threads (default: true) to hot threads

Adds ignore_idle_threads to hot threads request, defaulting to true.

It's somewhat heuristicky ... but seems to work.

Closes #8908
I wonder if the default value when the request comes from an older node should be `false` instead. I know that the default for the new flag is `true`, but given that the previous behaviour was `false`, when the request comes from an older node it should still be false, otherwise we'd get mixed results overall, some (from older nodes) that don't ignore idle threads, and some that do?