JsonRpcConnection: Don't read any data on shutdown

When the `Desconnect()` method is called, clients are not disconnected
immediately. Instead, a new coroutine is spawned using the same strand
as the other coroutines. This coroutine calls `async_shutdown` on the
TCP socket, which might be blocking. However, in order not to block
indefintely, the `Timeout` class cancels all operations on the socket
after `10` seconds. Though, the timeout does not trigger the handler
immediately; it creates spawns another coroutine using the same strand
as in the `JsonRpcConnection` class. This can cause unexpected delays if
e.g. `HandleIncomingMessages` gets resumed before the coroutine from the
timeout class. Apart from that, the coroutine for writing messages uses
the same condition, making the two symmetrical.

lib/remote/jsonrpcconnection.cpp

@@ -62,13 +62,13 @@ void JsonRpcConnection::HandleIncomingMessages(boost::asio::yield_context yc)
 {
 	m_Stream->next_layer().SetSeen(&m_Seen);
 
-	for (;;) {
+	while (!m_ShuttingDown) {
 		String message;
 
 		try {
 			message = JsonRpc::ReadMessage(m_Stream, yc, m_Endpoint ? -1 : 1024 * 1024);
 		} catch (const std::exception& ex) {
-			Log(m_ShuttingDown ? LogDebug : LogNotice, "JsonRpcConnection")
+			Log(LogNotice, "JsonRpcConnection")
 				<< "Error while reading JSON-RPC message for identity '" << m_Identity
 				<< "': " << DiagnosticInformation(ex);