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/util.h

@@ -15,6 +15,8 @@
 typedef int pid_t; /* define for Windows compatibility */
 #endif
 #include <map>
+#include <list>
+#include <utility>
 #include <vector>
 #include <string>
 
@@ -523,4 +525,3 @@ inline uint32_t ByteReverse(uint32_t value)
 }
 
 #endif
-