refactor & improvements

[eytan-avisror]

Contributes to #35

This PR refactors a lot of the code for better structure and better performance of API Calls to AWS.

• Logging improvements
• Deregistering improvements - calls are now made across instances instead of per target per group. (great performance improvement)
• Use inverse-exponential-backoff for waiters
• Improved error handling for goroutines
• caching improvements (don't flush on Deregister)

Testing Done:

• tested performance on 35 nodes concurrently killed on 250 tg, 0 errors, avg. 1500 seconds
• tested performance on 50 nodes concurrently killed on 250 tg, 0 errors, avg. 2800 seconds see test results in last comment

[codecov[bot]]

Codecov Report

Merging #42 into master will increase coverage by 0.24%. The diff coverage is 69.52%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   72.82%   73.06%   +0.24%     
==========================================
  Files           9       12       +3     
  Lines         861      995     +134     
==========================================
+ Hits          627      727     +100     
- Misses        176      206      +30     
- Partials       58       62       +4

Impacted Files	Coverage Δ
pkg/service/lifecycle.go	61.53% <ø> (ø)
pkg/service/nodes.go	86.58% <ø> (-0.17%)	:arrow_down:
pkg/service/elbv2.go	100% <100%> (ø)	:arrow_up:
pkg/service/events.go	78.57% <100%> (-2.68%)	:arrow_down:
pkg/service/elb.go	100% <100%> (ø)	:arrow_up:
pkg/service/metrics.go	84.61% <33.33%> (-15.39%)	:arrow_down:
pkg/service/deregistrator.go	47.88% <47.88%> (ø)
pkg/service/autoscaling.go	78.57% <60%> (ø)	:arrow_up:
pkg/service/server.go	62.82% <66.44%> (+0.46%)	:arrow_up:
pkg/service/target.go	78.26% <78.26%> (ø)
... and 4 more

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[eytan-avisror]

Load tests:

[eytan-avisror]

Can you add a "Testing Done" section to the commit? For large PRs, that's the easiest way to check if all corner cases have been covered.

There are test results one comment above, I've added a summary in the PR