input/Stream: remove attribute "cond", replace with handler interface

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

test/run_input.cxx

@@ -239,8 +239,7 @@ try {
 	/* open the stream and dump it */
 
 	Mutex mutex;
-	Cond cond;
-	auto is = InputStream::OpenReady(c.uri, mutex, cond);
+	auto is = InputStream::OpenReady(c.uri, mutex);
 	return dump_input_stream(is.get());
 } catch (const std::exception &e) {
 	LogError(e);