STASHDEV-7788 Change `BasicOperationScheduler.responseThread` from a singleton thread into a small thread pool.

This is intended to alleviate the observed bottleneck where (according to `HealthMonitor` stats on one of our
production instances) `executor.q.response.size` and `operations.remote.size` keep growing under heavy load.
Scaled back number of response threads from 2*cpus to 1*cpus.