Restart analysis if revision changes during validation

[stefanprodan]

Currently if there is an canary analysis underway and a new revision is applied on the cluster, Flagger will wait for the new revision to be rolled out and will resume the analysis. This could lead to an erroneous promotion since the new revision will not be fully validated. Instead of resuming the analysis Flagger should reset the validation process (set the failed checks and canary weight to zero) and start a fresh analysis.

Fix #30

[codecov-io]

Codecov Report

Merging #31 into master will increase coverage by 4.58%.
The diff coverage is 48.88%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage    49.1%   53.68%   +4.58%     
==========================================
  Files           8        8              
  Lines        1010     1030      +20     
==========================================
+ Hits          496      553      +57     
+ Misses        431      380      -51     
- Partials       83       97      +14

Impacted Files	Coverage Δ
pkg/controller/job.go	0% <0%> (ø)	⬆️
pkg/controller/deployer.go	61.83% <0%> (+4.09%)	⬆️
pkg/controller/scheduler.go	39.81% <55%> (+17.59%)	⬆️
pkg/controller/observer.go	64% <0%> (+4%)	⬆️

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