replay/framereader: no longer cache all AVPacket instances in memory

[deanlee]

Caching all AVPacket instances in memory can simplify the seeking to a specific frame, but it consumes significant memory resources, limits the scalability of the FrameReader. especially when caching a greater number of segments in Replay or Cabana. it can lead to increased memory usage, potentially causing performance issues or even system instability if memory resources are exhausted.

This PR only stores the pkt_pos of each frame in memory. it uses avio_seek to seek to the frame's position, greatly reducing the memory footprint of FrameReader. It saves nearly 100MB of memory per segment for the road camera, with even greater savings when playing multiple cameras.

It will be slightly slower than caching all AVPacket in memory, but since we have implemented frame prefetching in the camera thread, the performance gap is negligible.

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