Parallel the session migration process

[djshow832]

Development Task

Currently, the process of session migration is serial:

1. Query the session states
2. Find a backend
3. Connect to the backend and retry if it fails
4. Read the initial handshake from the new backend
5. Handshake with the new backend
6. Set the session states into the new connection
7. Disconnect the old connection

A long session migration may affect the query duration. We can do the 1st step while doing the 2nd, 3rd, and 4th steps. It may reduce the session migration duration.