Fix bug: 1.constant_cl for method doesn't work 2.qps = 0 in gradient_cl 3.SeverOptions.max_concurrency can't return correct value when using auto_cl

delete Describe() from ConcurrencyLimiter, Optimize the cacheline alignment of the Gradient ConcurrencyLimiter, fix code style

1. Set the default value of cl to NULL  2. Modify the interface such as MaxConcurrencyOf, and the call will be given an error after the service is started.

merge from baidu, fix some typo and bvar bug

- fix divide by zero error caused by output struct Stat while changing by sampling thread

- fix divide by zero error caused by output struct Stat while changing by sampling thread