Swaps mutex for atomic in ingester's OnceSwitch

[owen-d]

benchmark                  old ns/op     new ns/op     delta
Benchmark_OnceSwitch-8     13418         5743          -57.20%

[slim-bean]

LGTM

[codecov-io]

Codecov Report

Merging #3141 (44b5e10) into master (ed649ee) will increase coverage by 0.02%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #3141      +/-   ##
==========================================
+ Coverage   63.11%   63.13%   +0.02%     
==========================================
  Files         188      188              
  Lines       16312    16305       -7     
==========================================
- Hits        10295    10294       -1     
+ Misses       5078     5069       -9     
- Partials      939      942       +3     

Impacted Files	Coverage Δ
pkg/ingester/instance.go	60.28% <80.00%> (-1.31%)	⬇️
pkg/querier/queryrange/limits.go	91.66% <0.00%> (-4.17%)	⬇️
pkg/logql/evaluator.go	90.23% <0.00%> (+0.35%)	⬆️
pkg/promtail/targets/file/filetarget.go	64.33% <0.00%> (+0.69%)	⬆️
pkg/canary/comparator/comparator.go	78.18% <0.00%> (+1.81%)	⬆️
pkg/querier/queryrange/downstreamer.go	97.64% <0.00%> (+2.35%)	⬆️