Refactor the ActiveAck implemenation to simplify testing

[chetanmeh]

This is based on discussion in #4641 and to simplify adding test for changes to be introduced in #4632

Description

This PR refactors out the logic related to active in a separate class which enables implementing unit test for the same. This also abstracts out the user event flow such that it can be tested independently of its default configuration

There is no functional change in this PR

Related issue and scope

• I opened an issue to propose and discuss this change (#????)

My changes affect the following components

• API
• Controller
• Message Bus (e.g., Kafka)
• Loadbalancer
• Invoker
• Intrinsic actions (e.g., sequences, conductors)
• Data stores (e.g., CouchDB)
• Tests
• Deployment
• CLI
• General tooling
• Documentation

Types of changes

• Bug fix (generally a non-breaking change which closes an issue).
• Enhancement or new feature (adds new functionality).
• Breaking change (a bug fix or enhancement which changes existing behavior).

Checklist:

• I signed an Apache CLA.
• I reviewed the style guides and followed the recommendations (Travis CI will check :).
• I added tests to cover my changes.
• My changes require further changes to the documentation.
• I updated the documentation where necessary.

[codecov-io]

Codecov Report

Merging #4658 into master will decrease coverage by 4.7%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #4658      +/-   ##
==========================================
- Coverage   83.12%   78.41%   -4.71%     
==========================================
  Files         184      185       +1     
  Lines        8438     8442       +4     
  Branches      591      593       +2     
==========================================
- Hits         7014     6620     -394     
- Misses       1424     1822     +398

Impacted Files	Coverage Δ
...pache/openwhisk/core/invoker/InvokerReactive.scala	79.46% <100%> (+21.61%)	⬆️
...he/openwhisk/core/invoker/MessagingActiveAck.scala	53.84% <53.84%> (ø)
...core/database/cosmosdb/RxObservableImplicits.scala	0% <0%> (-100%)	⬇️
...ore/database/cosmosdb/cache/CacheInvalidator.scala	0% <0%> (-100%)	⬇️
...core/database/cosmosdb/CosmosDBArtifactStore.scala	0% <0%> (-96.17%)	⬇️
...tabase/cosmosdb/cache/CacheInvalidatorConfig.scala	0% <0%> (-94.74%)	⬇️
...sk/core/database/cosmosdb/CosmosDBViewMapper.scala	0% <0%> (-92.6%)	⬇️
...e/database/cosmosdb/cache/ChangeFeedListener.scala	0% <0%> (-86.67%)	⬇️
...e/database/cosmosdb/cache/KafkaEventProducer.scala	0% <0%> (-76.48%)	⬇️
...whisk/core/database/cosmosdb/CosmosDBSupport.scala	0% <0%> (-74.08%)	⬇️
... and 32 more

---

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.41%. Comparing base (3f9a780) to head (0b0f874).
⚠️ Report is 489 commits behind head on master.

Files with missing lines	Patch %	Lines
...he/openwhisk/core/invoker/MessagingActiveAck.scala	53.84%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4658      +/-   ##
==========================================
- Coverage   83.12%   78.41%   -4.71%     
==========================================
  Files         184      185       +1     
  Lines        8438     8442       +4     
  Branches      591      593       +2     
==========================================
- Hits         7014     6620     -394     
- Misses       1424     1822     +398     

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