RFC: Robust stable connection.

[sunhuachuang]

Now stable has two connect type.

• Direct (EndpointStream) - listen the transport's stream. like Tcpstream.
• Relay (SessionSend) - use other session's channel address as endpoint.

If listen, need handle two way.

• build stable connection table, add other connect type and keep stable connect the remote peer. (by relay).
• If current connection is lost, use other channel to it. and then rebuild stable connection table.

If lost connection.

• If now is Directly, reconnect. (if not ok, change to Relay way.).
• If now is Relay, if relay session is closed, choose other closest peer in DHT and send Ping to check.