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>
joinAll will wait for all threads that have been created in the pool, you can create them on different threads and they will be added to the total.
You should be careful to avoid [re-]creating the pool and [re-]joining the pool from more than one thread though.