bcos节点入网以后，与peer端建立session，然后初始化BlockChainSync,并运行如下状态机，进行节点数据下载：

NotSynced,			///< Initial chain sync has not started yet
Idle,				///< Initial chain sync complete. Waiting for new packets
Waiting,			///< Block downloading paused. Waiting for block queue to process blocks and free space
Blocks,				///< Downloading blocks
State,				///< Downloading state
NewBlocks,			///< Downloading blocks learned from NewHashes packet

bcos节点数据下载报文交互类型：

  StatusPacket = 0x00,
  NewBlockHashesPacket = 0x01,
  TransactionsPacket = 0x02,
  GetBlockHeadersPacket = 0x03,
  BlockHeadersPacket = 0x04,
  GetBlockBodiesPacket = 0x05,
  BlockBodiesPacket = 0x06,
  NewBlockPacket = 0x07,

  GetNodeDataPacket = 0x0d,
  NodeDataPacket = 0x0e,
  GetReceiptsPacket = 0x0f,
  ReceiptsPacket = 0x10,