HPCC-12001 Improve performance for WsWorkunits.WUListQueries

To get Query status on clusters, WsWorkunits.WUListQueries has to
send <control:queries/> calls to each roxie cluster. This fix
creates a thread pool to send the calls in parallel.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>
HPCC-12001 Revise based on review comments

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>
HPCC-12001 Fix a typo and add more comment

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>
Merge pull request #6295 from wangkx/thread_list_queries

HPCC-12001 Improve performance for WsWorkunits.WUListQueries

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>