Feature Request: Rewrite Select for Lock-related Functions（engine.Lock）

[earayu]

Background:

The Select function for lock-related functions generates an engine.Lock type of plan and has a corresponding execution function. However, there is still room for improvement in terms of their correctness and compatibility with MySQL.

Solution:

We need to rewrite engine.Lock primitive and related planner. The VTGate layer need not to evaluate expressions or split the locking functions. Just like engine.Send, VTGate should send the SQL to VTTablet, and add some flags in session. That's all.