Prepare for switch to Scala 2.12.7

[chetanmeh]

This PR complements the #4062. It makes required code changes which also work with Scala 2.11. This can be committed sooner and would reduce any rework to be done in case it takes time to merge #4062.

This is specially important to minimize rework due to #4073. So even if #4073 get merged before Switch to Scala 2.12 it would be easier to do required leftover changes for #4062

Description

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 #4074 into master will decrease coverage by 4.94%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master    #4074      +/-   ##
==========================================
- Coverage      86%   81.05%   -4.95%     
==========================================
  Files         147      147              
  Lines        7180     7180              
  Branches      445      439       -6     
==========================================
- Hits         6175     5820     -355     
- Misses       1005     1360     +355

Impacted Files	Coverage Δ
...scala/whisk/core/mesos/MesosContainerFactory.scala	61.81% <ø> (ø)	⬆️
...on/scala/src/main/scala/whisk/common/Logging.scala	89.36% <0%> (-0.97%)	⬇️
...core/database/cosmosdb/CosmosDBArtifactStore.scala	0% <0%> (-95.54%)	⬇️
...cala/src/main/scala/whisk/core/entity/Limits.scala	84.61% <100%> (ø)	⬆️
...rc/main/scala/whisk/core/database/StoreUtils.scala	100% <100%> (ø)	⬆️
...n/scala/whisk/core/database/CouchDbRestStore.scala	73.23% <100%> (ø)	⬆️
...ala/whisk/core/database/s3/S3AttachmentStore.scala	85.71% <100%> (ø)	⬆️
...core/controller/BasicAuthenticationDirective.scala	100% <100%> (ø)	⬆️
...isk/core/database/memory/MemoryArtifactStore.scala	97.54% <83.33%> (-0.84%)	⬇️
...core/database/cosmosdb/RxObservableImplicits.scala	0% <0%> (-100%)	⬇️
... and 5 more

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