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/rpcwallet.cpp

@@ -1289,7 +1289,7 @@ void ThreadCleanWalletPassphrase(void* parg)
                 break;
 
             LEAVE_CRITICAL_SECTION(cs_nWalletUnlockTime);
-            Sleep(nToSleep);
+            MilliSleep(nToSleep);
             ENTER_CRITICAL_SECTION(cs_nWalletUnlockTime);
 
         } while(1);