JsonRpcConnection: don't flush any data on shutdown

In fact, this is already done for both loops (for each bulk and for each message of it). So once the remote signals EOF, don't try to get things done until write error (which can't be associated with a particular message anyway, due to buffering), but just let the peer go.

lib/remote/jsonrpcconnection.cpp

@@ -121,6 +121,10 @@ void JsonRpcConnection::WriteOutgoingMessages(boost::asio::yield_context yc)
 					}
 				}
 
+				if (m_ShuttingDown) {
+					return;
+				}
+
 				m_Stream->async_flush(yc);
 			} catch (const std::exception& ex) {
 				Log(m_ShuttingDown ? LogDebug : LogWarning, "JsonRpcConnection")