Support older versions of Go

[elgopher]

Is your feature request related to a problem? Please describe.
Currenty yala requires Go 1.17. But some developers use older versions. It would be nice to support Go versions starting with 1.15 (at least)

Describe the solution you'd like
Yala uses atomic.Value.CompareAndSwap() method which requires Go 1.17. Maybe this method can be ported to library itself?

Describe alternatives you've considered
Maybe use different atomic struct for storing logger inside logger.Global?