Set StableRS hash to current if replicaset does not actually exist

[dthomson25]

Resolves #305

[codecov]

Codecov Report

Merging #320 into master will increase coverage by 0.24%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
+ Coverage   83.89%   84.13%   +0.24%     
==========================================
  Files          66       66              
  Lines        6028     6027       -1     
==========================================
+ Hits         5057     5071      +14     
+ Misses        698      683      -15     
  Partials      273      273

Impacted Files	Coverage Δ
rollout/sync.go	76.04% <100%> (-0.07%)	⬇️
rollout/context.go	96.07% <100%> (-0.04%)	⬇️
utils/replicaset/canary.go	79.71% <100%> (+11.32%)	⬆️
rollout/canary.go	86.23% <83.33%> (-0.06%)	⬇️

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