Fix pulse update on backend removal

[sebbonnet]

Remove the pulse info from the stash when a backend is removed. This is to avoid the scenario where an unhealthy backend stored in the stash remains unhealthy after being scaled down and up again. In this case the pulse update had no effect as the removed backend was still found in the stash.

Fix made in 2 commits to better visualise the actual change in behaviour

[codecov-io]

Codecov Report

Merging #48 into master will increase coverage by 6.01%. The diff coverage is 70%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   33.39%   39.41%   +6.01%     
==========================================
  Files          18       18              
  Lines        1018     1030      +12     
==========================================
+ Hits          340      406      +66     
+ Misses        656      594      -62     
- Partials       22       30       +8

Impacted Files	Coverage Δ
pulse/status.go	54.54% <0%> (-34.35%)	:arrow_down:
pulse/pulse.go	96.87% <100%> (+0.1%)	:arrow_up:
core/runtime.go	60.71% <72.34%> (+60.71%)	:arrow_up:
core/context.go	20.2% <0%> (+8.33%)	:arrow_up:

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[sebbonnet]

@kobolog would you mind taking a look?

[kobolog]

Looks good!

[kobolog]

Thanks for the good work!