Reduce number of Helix threads in Pinot server

In numerous profiles collected from different deployments, the number of `HelixTaskExecutor-message_handle_thread` is always 42, even in small deployments with 4 cores and 4GB heap per server. This number seems far too high and does not depend on the number of cores available, so should be reduced to depend on the number of cores available to the server.

The issue identifies HelixTaskExecutor-message_handle_thread in the Pinot server; start by locating where this thread count is configured or created. Compare the current fixed count with the server's available cores, then verify on a small deployment that the number of Helix threads scales with cores without affecting server operation.

Written by the indexing model from the issue text.