boost: drop boost threads for [alert|block|wallet]notify

src/init.cpp

@@ -546,7 +546,8 @@ static void BlockNotifyCallback(bool initialSync, const CBlockIndex *pBlockIndex
     std::string strCmd = gArgs.GetArg("-blocknotify", "");
     if (!strCmd.empty()) {
         boost::replace_all(strCmd, "%s", pBlockIndex->GetBlockHash().GetHex());
-        boost::thread t(runCommand, strCmd); // thread runs free
+        std::thread t(runCommand, strCmd);
+        t.detach(); // thread runs free
     }
 }