revert stop_daemon method to use correct exit

This was changed because sometimes the daemon does not complete its exit
routine with this method, but as it does correctly wind most things down
even if it gets stuck I've changed it back.

src/rpc/core_rpc_server.cpp

@@ -411,7 +411,7 @@ namespace cryptonote
   {
     // FIXME: replace back to original m_p2p.send_stop_signal() after
     // investigating why that isn't working quite right.
-    m_core.stop();
+    m_p2p.send_stop_signal();
     res.status = CORE_RPC_STATUS_OK;
     return true;
   }