Fix for bug 3258. We now maintain a pool of N-1 generic worker threads which is extended by one client-specific worker thread for each client that enters the dataflow code.
Fix for bug 3258.
We now maintain a pool of N-1 generic worker threads which is extended
by one client-specific worker thread for each client that enters the
dataflow code.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb