validate max_background_jobs will fail when cpu quota is less than 1

[CalvinNeo]

Bug Report

What version of TiKV are you using?

master

What operating system and CPU are you using?

all

Steps to reproduce

when cpu quota is less than 1

What did you expect?

Not sure.

What did happened?

According to https://github.com/tikv/tikv/blob/master/src/config.rs#L1240, if cpu quota is less than 1, we may get a validation error.

[FATAL] [[config.rs:400](http://config.rs:400/)] ["invalid configuration: max_background_jobs should be greater than 0 and less than or equal to 0"]

Is this by design?