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>
Question about joinAll(): if there are two calls to a thread pool at the same time, the first one creates m threads and the second one creates n threads. Does the joinAll() wait for m or n threads (depending on which call) to finish, or wait for m+n threads to finish?