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/net.h

@@ -75,7 +75,6 @@ enum threadId
     THREAD_SOCKETHANDLER,
     THREAD_OPENCONNECTIONS,
     THREAD_MESSAGEHANDLER,
-    THREAD_MINER,
     THREAD_RPCLISTENER,
     THREAD_UPNP,
     THREAD_DNSSEED,