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/EmbeddedCuePlaylistPlugin.cxx

@@ -89,8 +89,7 @@ static constexpr TagHandler embcue_tag_handler = {
 
 static std::unique_ptr<SongEnumerator>
 embcue_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 */

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 */

src/playlist/plugins/SoundCloudPlaylistPlugin.cxx

@@ -221,9 +221,9 @@ static constexpr yajl_callbacks parse_callbacks = {
  */
 static void
 soundcloud_parse_json(const char *url, Yajl::Handle &handle,
-		      Mutex &mutex, Cond &cond)
+		      Mutex &mutex)
 {
-	auto input_stream = InputStream::OpenReady(url, mutex, cond);
+	auto input_stream = InputStream::OpenReady(url, mutex);
 	Yajl::ParseInputStream(handle, *input_stream);
 }
 
@@ -235,7 +235,7 @@ soundcloud_parse_json(const char *url, Yajl::Handle &handle,
  *	soundcloud://url/<url or path of soundcloud page>
  */
 static std::unique_ptr<SongEnumerator>
-soundcloud_open_uri(const char *uri, Mutex &mutex, Cond &cond)
+soundcloud_open_uri(const char *uri, Mutex &mutex)
 {
 	assert(strncmp(uri, "soundcloud://", 13) == 0);
 	uri += 13;
@@ -277,7 +277,7 @@ soundcloud_open_uri(const char *uri, Mutex &mutex, Cond &cond)
 
 	SoundCloudJsonData data;
 	Yajl::Handle handle(&parse_callbacks, nullptr, &data);
-	soundcloud_parse_json(u, handle, mutex, cond);
+	soundcloud_parse_json(u, handle, mutex);
 
 	data.songs.reverse();
 	return std::make_unique<MemorySongEnumerator>(std::move(data.songs));