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/init.h

@@ -7,11 +7,12 @@
 
 #include "wallet.h"
 
+class boost::thread_group;
 extern CWallet* pwalletMain;
 
 void StartShutdown();
 void Shutdown(void* parg);
-bool AppInit2();
+bool AppInit2(boost::thread_group& threadGroup);
 std::string HelpMessage();
 
 #endif