hlssink: implement a chain_list to request key unit for segments

hlssink: implement a chain_list to request key unit for segments

upstream might send buffer lists instead of buffers and hlssink's
probe won't get called and a new segment won't be created when needed.

This patch fixes it by adding a chain_list function to the sink pad
that will just pass through the whole bufferlist if no segment needs
to be requested at the moment or convert the list into buffers to
check the proper timestamp to request the next key-unit that will
start the segment.

https://bugzilla.gnome.org/show_bug.cgi?id=746906