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>
Is it because there are threads already and you only want to join the ones you just created?