input_stream: non-blocking I/O

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

src/decoder/mp4ff_decoder_plugin.c

@@ -102,7 +102,8 @@ mp4_seek(void *user_data, uint64_t position)
 {
 	struct mp4ff_input_stream *mis = user_data;
 
-	return input_stream_seek(mis->input_stream, position, SEEK_SET, NULL)
+	return input_stream_lock_seek(mis->input_stream, position, SEEK_SET,
+				      NULL)
 		? 0 : -1;
 }