Improve automatic tuning of thread pool:

The job queue can automatically tune the number of threads that
it creates based on the number of processors or processor cores
that are available.

The existing tuning was very conservative, limiting the maximum
number of threads to only 6.

Adjust the new algorithm to allow a larger number of threads and
allow server administrators to override the value in the config
file.

src/ripple/core/JobQueue.h

@@ -163,7 +163,8 @@ public:
 
     /** Set the number of thread serving the job queue to precisely this number.
     */
-    void setThreadCount (int c, bool const standaloneMode);
+    void setThreadCount (int c, bool const standaloneMode,
+                         bool const validator=true);
 
     /** Return a scoped LoadEvent.
     */