daemon: stop miner before we bring the whole thing down

This avoids the miner erroring out trying to submit blocks
to a core that's already shut down (and avoids pegging
the CPU while we're busy shutting down).

src/cryptonote_core/cryptonote_core.cpp

@@ -247,6 +247,7 @@ namespace cryptonote
   //-----------------------------------------------------------------------------------
   void core::stop()
   {
+    m_miner.stop();
     m_blockchain_storage.cancel();
 
     tools::download_async_handle handle;