重传列表的最大限制是500个包，大约10秒钟
// A limit for the size of the NACK list.
static const size_t kNackListSizeLimit = 500; // 10 seconds for 20 ms frame
// packets.
允许设置一个小于max_nack_list_size的值
// Set a maximum for the size of the NACK list. If the last received packet
// has sequence number of N, then NACK list will not contai