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/PlaylistAny.cxx

@@ -29,13 +29,13 @@ playlist_open_any(const char *uri,
 #ifdef ENABLE_DATABASE
 		  const Storage *storage,
 #endif
-		  Mutex &mutex, Cond &cond)
+		  Mutex &mutex)
 {
 	return uri_has_scheme(uri)
-		? playlist_open_remote(uri, mutex, cond)
+		? playlist_open_remote(uri, mutex)
 		: playlist_mapper_open(uri,
 #ifdef ENABLE_DATABASE
 				       storage,
 #endif
-				       mutex, cond);
+				       mutex);
 }