Roxie worker threads are given elevated priority if started on demand

Since the addition of the prestartSlaveThreads option in 6.0 , if it is NOT set then Roxie agent threads will be started by the socket reading thread, the first time a query is received. However, as this thread runs at elevated priority, the worker threads (and their children) will also run elevated. This is not what was intended nor is it desirable.

The prestart option defaults to true in the config xsd, but perhaps has been overridden (and probably won't be set on systems that were first created prior to 6.x).