HPCC-9303 - Use thread pool for DFS requests

Use threadpool for DFS server (used to be single threaded)

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4406 from jakesmith/hpcc-9303

HPCC-9303 - Use thread pool for DFS requests

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>