Support graceful shutdown of runtime pods.

[style95]

Description

This is to support the graceful shutdown.
When OW is running on K8S and a pod is drained for some reason (server maintenance..)
Our runtime pod also shut down and a time-out error can occur.

In this case, activation's result is an action developer error but it's actually the internal system error.
So this is to handle such a case.

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
• Scheduler
• 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-commenter]

Codecov Report

Merging #5283 (f1dca88) into master (4016aa8) will decrease coverage by 4.62%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5283      +/-   ##
==========================================
- Coverage   79.96%   75.33%   -4.63%     
==========================================
  Files         238      238              
  Lines       14122    14129       +7     
  Branches      599      606       +7     
==========================================
- Hits        11292    10644     -648     
- Misses       2830     3485     +655     

Impacted Files	Coverage Δ
...pache/openwhisk/core/containerpool/Container.scala	90.32% <100.00%> (+0.66%)	⬆️
...pache/openwhisk/core/entity/ActivationResult.scala	90.66% <100.00%> (+0.12%)	⬆️
...core/database/cosmosdb/RxObservableImplicits.scala	0.00% <0.00%> (-100.00%)	⬇️
...ore/database/cosmosdb/cache/CacheInvalidator.scala	0.00% <0.00%> (-100.00%)	⬇️
...e/database/cosmosdb/cache/ChangeFeedConsumer.scala	0.00% <0.00%> (-100.00%)	⬇️
...core/database/cosmosdb/CosmosDBArtifactStore.scala	0.00% <0.00%> (-95.85%)	⬇️
...sk/core/database/cosmosdb/CosmosDBViewMapper.scala	0.00% <0.00%> (-93.90%)	⬇️
...tabase/cosmosdb/cache/CacheInvalidatorConfig.scala	0.00% <0.00%> (-92.31%)	⬇️
...enwhisk/connector/kafka/KamonMetricsReporter.scala	0.00% <0.00%> (-83.34%)	⬇️
...e/database/cosmosdb/cache/KafkaEventProducer.scala	0.00% <0.00%> (-78.58%)	⬇️
... and 16 more

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