Apply `pd_enable_follower_handle_region` to Lightning/BR

[okJiang]

Enhancement

pd_enable_follower_handle_region only works on tidb now. We want to apply it to Lightning/BR who may request too many GetRegion/ScanRegions to PD.

[okJiang]

After testing, I found this pr doesn't work. TiDB variable(pd_enable_follower_handle_region) can't affect BR/Lightning process directly. We need to get the variable in BR/Lightning and then set the PD client option.

PS: The previous PR will not affect BR/Lightning.