Merge topic branch t/btr into next

Some helpers such as btr_get_input_queue_size() are very expensive
if the buffer tree contains many buffers, which is the case for mp3
and aac streams. This is noticeable in particular with para_play,
which spends more time in the btr subsystem than in the mp3 decoder
if the file is large enough. This branch contains a few patches to
improve that.

* refs/heads/t/btr:
  btr: Streamline the documentation of buffer_tree.h.
  Constify buffer tree API.
  btr: Merge buffers on insertion.
  btr: Speed up btr_node_status().
  Speed up mp3dec filter.