This is how the ref list manager links bitstream IDs to H264Picture/Ref
objects, and is local to the producer thread. There is no need for the
consumer thread to know the bitstream IDs of its references in their
respective producer threads.

In practice, this fixes tsan warnings when running fate-h264:

WARNING: ThreadSanitizer: data race (pid=19295)
  Read of size 4 at 0x7dbc0000e614 by main thread (mutexes: write M1914):
    #0 ff_h264_ref_picture src/libavcodec/h264_picture.c:112 (ffmpeg+0x0000013b3709)
[..]
  Previous write of size 4 at 0x7dbc0000e614 by thread T2 (mutexes: write M1917):
    #0 build_def_list src/libavcodec/h264_refs.c:91 (ffmpeg+0x0000013b46cf)