HPCC-9385 - Fix thread pool runningCount()

runningCount() was incorrectly tracked
Add optional tracing for pool thread starts and average delay

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4422 from jakesmith/hpcc-9385

HPCC-9385 - Fix thread pool runningCount()

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>