Use real number of cores for default -par, ignore virtual cores

To determine the default for `-par`, the number of script verification
threads, use [boost::thread::physical_concurrency()](http://www.boost.org/doc/libs/1_58_0/doc/html/thread/thread_management.html#thread.thread_management.thread.physical_concurrency)
which counts only physical cores, not virtual cores.

Virtual cores are roughly a set of cached registers to avoid context
switches while threading, they cannot actually perform work, so spawning
a verification thread for them could even reduce efficiency and will put
undue load on the system.

Should fix issue #6358, as well as some other reported system overload
issues, especially on Intel processors.

The function was only introduced in boost 1.56, so provide a utility
function `GetNumCores` to fall back for older Boost versions.

src/util.cpp

@@ -756,3 +756,13 @@ void SetThreadPriority(int nPriority)
 #endif // PRIO_THREAD
 #endif // WIN32
 }
+
+int GetNumCores()
+{
+#if BOOST_VERSION >= 105600
+    return boost::thread::physical_concurrency();
+#else // Must fall back to hardware_concurrency, which unfortunately counts virtual cores
+    return boost::thread::hardware_concurrency();
+#endif
+}
+