Gracefully check liveness of a backend connection

[djshow832]

Development Task

In #198, the PR checks the liveness of a backend connection by peeking one byte. There are some disadvantages:

• It costs 1ms to check
• It wraps the connection with another buffer, which makes it even heavier

I found that in go-sql-driver, there's a smarter way to do this: go-sql-driver/mysql#934
It solves the above 2 problems and it must be stable because it runs in go driver for 4 years.