MCOL-1810 Fix hang on low core count

Anything that links against joblist will spin up a threadpool upon
startup. This includes the tools setConfig/getConfig. It is possible on
a low core count machine or low CPU speed that the signal to the prune
thread to shutdown is sent before the thread has completed startup when
these quick-running tools are used.

This fix adds a mutex so that spin up and shutdown can't happen at the
same time as well as a stop watch in case we are shutting down when
either the thread is running or we haven't fully started.

utils/threadpool/threadpool.h

@@ -357,6 +357,7 @@ private:
 
     std::string fName;  // Optional to add a name to the pool for debugging.
     bool fDebug;
+    boost::mutex fInitMutex;
     boost::mutex fPruneMutex;
     boost::condition fPruneThreadEnd;
     boost::thread* fPruneThread;