Shutdown cleanup prep-work

Create a boost::thread_group object at the qt/bitcoind main-loop level
that will hold pointers to all the main-loop threads.

This will replace the vnThreadsRunning[] array.

For testing, ported the BitcoinMiner threads to use its
own boost::thread_group.

src/qt/bitcoin.cpp

@@ -11,6 +11,7 @@
 #include "guiutil.h"
 #include "guiconstants.h"
 #include "init.h"
+#include "util.h"
 #include "ui_interface.h"
 #include "paymentserver.h"
 
@@ -215,9 +216,10 @@ int main(int argc, char *argv[])
         if (GUIUtil::GetStartOnSystemStartup())
             GUIUtil::SetStartOnSystemStartup(true);
 
+        boost::thread_group threadGroup;
         BitcoinGUI window;
         guiref = &window;
-        if(AppInit2())
+        if(AppInit2(threadGroup))
         {
             {
                 // Put this in a block, so that the Model objects are cleaned up before
@@ -259,6 +261,8 @@ int main(int argc, char *argv[])
             }
             // Shutdown the core and its threads, but don't exit Bitcoin-Qt here
             Shutdown(NULL);
+            threadGroup.interrupt_all();
+            threadGroup.join_all();
         }
         else
         {