simplify sticky balancer and fix a bug

[ElvinEfendi]

What this PR does / why we need it:

• Simplify sticky.lua
• Fix a bug in the feature introduced at Change upstream on error when sticky session balancer is used #4048. The problem is the code looks at last failing upstream only. However if Nginx is configured to retry more than 2 upstreams then it is possible that sticky balancer picks a previously failing upstream again. This PR makes sure sticky balancer looks at all previously failing upstreams for given request and does not pick any of them as a new upstream. cc @fedunineyu

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

[codecov-io]

Codecov Report

Merging #4169 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4169      +/-   ##
==========================================
+ Coverage   57.63%   57.66%   +0.03%     
==========================================
  Files          87       87              
  Lines        6463     6463              
==========================================
+ Hits         3725     3727       +2     
+ Misses       2307     2306       -1     
+ Partials      431      430       -1

Impacted Files	Coverage Δ
internal/ingress/metric/collectors/process.go	90.42% <0%> (+2.12%)	⬆️

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[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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