Rename util.h Sleep --> MilliSleep

Two reasons for this change:
1. Need to always use boost::thread's sleep, even on Windows, so the
sleeps can be interrupted (prior code used Windows' built-in Sleep).

2. I always forgot what units the old Sleep took.

src/bitcoinrpc.cpp

@@ -1003,7 +1003,7 @@ void ThreadRPCServer3(void* parg)
                If this results in a DOS the user really
                shouldn't have their RPC port exposed.*/
             if (mapArgs["-rpcpassword"].size() < 20)
-                Sleep(250);
+                MilliSleep(250);
 
             conn->stream() << HTTPReply(HTTP_UNAUTHORIZED, "", false) << std::flush;
             break;