Refactor prepare_frame_packet

[tianluyuan]

The function prepare_frame_packet in extract_json_message.py consists of many moving parts and changes to state. One example, mentioned in #244 is the call to prepare_frames(frame_packet, ...) which modifies and returns an updated list of frames. We should try to simplify / clean up this function.

[ric-evans]

link: #244 (comment)

[mlincett]

I had outlined some ideas about this in #149 , but then abandoned that path.