lavc: Make AVPacket.duration int64, and deprecate convergence_duration

lavc: Make AVPacket.duration int64, and deprecate convergence_duration

Note that convergence_duration had another meaning, one which was in
practice never used. The only real use for it was a 64 bit replacement
for the duration field. It's better just to make duration 64 bits, and
to get rid of it.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>