avformat/flvenc: add add_keyframe_index option

      Add keyframe index metadata
Used to facilitate seeking; particularly for HTTP pseudo streaming.
 1. read live streaming or file by sequence
 2. if use add_keyframe_index option, add a mark flag at the position,
    use to insert new context at the last step.
 3. add the keyframes *offset* and *timestamp* into a list
 4. if use add_keyframe_index option, shift the metadata data from
    mark flag offset
 5. insert the keyframes *offset* and *timestamp* from the list by
    sequence
 6. free the list
 7. end.

Add FATE test case;

Reviewed-by: Lou Logan <lou@lrcd.com>
Signed-off-by: Steven Liu <liuqi@gosun.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>