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/DumpDecoderClient.cxx

@@ -73,7 +73,7 @@ DumpDecoderClient::SeekError()
 InputStreamPtr
 DumpDecoderClient::OpenUri(const char *uri)
 {
-	return InputStream::OpenReady(uri, mutex, cond);
+	return InputStream::OpenReady(uri, mutex);
 }
 
 size_t