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.

src/playlist/plugins/FlacPlaylistPlugin.cxx

@@ -88,7 +88,7 @@ FlacPlaylist::NextSong()
 
 static std::unique_ptr<SongEnumerator>
 flac_playlist_open_uri(const char *uri,
-		       gcc_unused Mutex &mutex, gcc_unused Cond &cond)
+		       gcc_unused Mutex &mutex)
 {
 	if (!PathTraitsUTF8::IsAbsolute(uri))
 		/* only local files supported */