client/Response: add constant MAX_BINARY_SIZE

Use the same chunk size for all binary commands.

src/client/Response.cxx

@@ -59,6 +59,8 @@ Response::Format(const char *fmt, ...) noexcept
 bool
 Response::WriteBinary(ConstBuffer<void> payload) noexcept
 {
+	assert(payload.size <= MAX_BINARY_SIZE);
+
 	return Format("binary: %zu\n", payload.size) &&
 		Write(payload.data, payload.size) &&
 		Write("\n");