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/init.cpp

@@ -43,7 +43,7 @@ enum BindFlags {
 void ExitTimeout(void* parg)
 {
 #ifdef WIN32
-    Sleep(5000);
+    MilliSleep(5000);
     ExitProcess(0);
 #endif
 }
@@ -105,7 +105,7 @@ void Shutdown(void* parg)
         UnregisterWallet(pwalletMain);
         delete pwalletMain;
         NewThread(ExitTimeout, NULL);
-        Sleep(50);
+        MilliSleep(50);
         printf("Bitcoin exited\n\n");
         fExit = true;
 #ifndef QT_GUI
@@ -116,8 +116,8 @@ void Shutdown(void* parg)
     else
     {
         while (!fExit)
-            Sleep(500);
-        Sleep(100);
+            MilliSleep(500);
+        MilliSleep(100);
         ExitThread(0);
     }
 }
@@ -1061,7 +1061,7 @@ bool AppInit2(boost::thread_group& threadGroup)
     // Loop until process is exit()ed from shutdown() function,
     // called from ThreadRPCServer thread when a "stop" command is received.
     while (1)
-        Sleep(5000);
+        MilliSleep(5000);
 #endif
 
     return true;