rtp序号和时间戳关联

// Records the association of RTP sequence numbers to timestamps and to whether
// the packet was first and/or last in the frame.
//
// 1. Limits number of entries. Whenever |max_entries| is about to be exceeded,
//    the size is reduced by approximately 25%.
// 2. RTP sequence numbers wrap around relatively infrequently.
//    This class therefore only remembers at most the last 2^15 RTP packets,
//    so that the newest packet's sequence number is still AheadOf the oldest
//    packet's sequence number.
// 3. Media f