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/db.cpp

@@ -413,7 +413,7 @@ bool CDB::Rewrite(const string& strFile, const char* pszSkip)
                 return fSuccess;
             }
         }
-        Sleep(100);
+        MilliSleep(100);
     }
     return false;
 }