RFC: message delivery feedback.

[sunhuachuang]

Why

Because p2p network is complex and unstable, so , add delivery feedback for stable connection.

How

• use u32 as data's symbol. and only work on stable connection, not DHT.
• Add new enum type ReceiveMessage::Delivery(u32, bool)
• Add u32 to ReceiveMessage::StableConnect(u32, ..), ReceiveMessage::StableResult(u32, ..), ReceiveMessage::Data(u32, ..)
• Add u32 to SendMessage::StableConnect(u32, ..), SendMessage::StableResult(u32, ...), SendMessage::Data(u32, ..)

[sunhuachuang]

Ok, Test on relay example.