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/walletdb.cpp

@@ -500,7 +500,7 @@ void ThreadFlushWalletDB(void* parg)
     int64 nLastWalletUpdate = GetTime();
     while (!fShutdown)
     {
-        Sleep(500);
+        MilliSleep(500);
 
         if (nLastSeen != nWalletDBUpdated)
         {
@@ -579,7 +579,7 @@ bool BackupWallet(const CWallet& wallet, const string& strDest)
                 }
             }
         }
-        Sleep(100);
+        MilliSleep(100);
     }
     return false;
 }