Replace usage of sync/atomic with uber-go/atomic

[jvrplmlmn]

What this PR does / why we need it: Replaces all direct usages of sync/atomic with go.uber.org/atomic. That way we ensure that all access to variables that have atomic access is in fact always atomic.

Which issue(s) this PR fixes:
Partially addresses #2448

Special notes for your reviewer: This only updates the code to use the alternative package. I haven't added any automation/linter to make sure that this is enforced.
For that, we could use faillint, but I am not 100% sure of where this project is handling the installation of additional tools. Is that on loki-build-image/Dockerfile?

Checklist

• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #2449 into master will decrease coverage by 0.13%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2449      +/-   ##
==========================================
- Coverage   62.91%   62.77%   -0.14%     
==========================================
  Files         162      162              
  Lines       13952    13952              
==========================================
- Hits         8778     8759      -19     
- Misses       4491     4510      +19     
  Partials      683      683              

Impacted Files	Coverage Δ
pkg/distributor/distributor.go	78.80% <60.00%> (ø)
pkg/ingester/mapper.go	90.36% <100.00%> (ø)
pkg/promtail/positions/positions.go	46.49% <0.00%> (-13.16%)	⬇️
pkg/promtail/targets/file/tailer.go	76.13% <0.00%> (-2.28%)	⬇️
pkg/logql/evaluator.go	92.47% <0.00%> (-0.41%)	⬇️

[owen-d]

Hey, thanks for this -- it looks great. I agree using faillint will help maintain these changes in the future.

Feel free to installl it here: https://github.com/grafana/loki/blob/master/loki-build-image/Dockerfile#L10-L12

and then reference in the make lint target here https://github.com/grafana/loki/blob/master/Makefile#L241

[sandeepsukhani]

LGTM! Thanks for the PR!
Regarding faillint, you are right about using loki-build-image/Dockerfile for setting up additional tools.
You can check PR in Cortex for reference. cortexproject/cortex#2272

[jvrplmlmn]

Thanks for the pointers @owen-d @sandeepsukhani.

It was not possible to add it to the golangci build image because it doesn't have the go toolchain in order to install the binary, so I created its own step.

I am not that familiar with the structure of this particular Makefile so let me know if the lint target was the right place to add the linter.

[owen-d]

lgtm, thanks!