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/main.cpp

@@ -4520,7 +4520,7 @@ void static BitcoinMiner(CWallet *pwallet)
 
     try { loop {
         while (vNodes.empty())
-            Sleep(1000);
+            MilliSleep(1000);
 
         //
         // Create new block