Sort Splunk logs by timestamp

[selfxp]

Fixes an issue that was noticed when pulling logs from Splunk where logs were coming in unexpected order. This PR will force logs to be sorted by timestamp first and then returned in a reverse order.

My changes affect the following components

• [ ] API
• [x] 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

• [x] 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:

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

[codecov-io]

Codecov Report

Merging #5057 (2854f93) into master (efdbd60) will increase coverage by 36.31%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master    #5057       +/-   ##
===========================================
+ Coverage   34.77%   71.09%   +36.31%     
===========================================
  Files         200      200               
  Lines        9759     9759               
  Branches      441      441               
===========================================
+ Hits         3394     6938     +3544     
+ Misses       6365     2821     -3544     

Impacted Files	Coverage Δ
...sk/core/containerpool/logging/SplunkLogStore.scala	85.18% <100.00%> (+85.18%)	:arrow_up:
...che/openwhisk/core/invoker/LogStoreCollector.scala	0.00% <0.00%> (-100.00%)	:arrow_down:
...a/org/apache/openwhisk/http/BasicHttpService.scala	3.33% <0.00%> (-86.67%)	:arrow_down:
...la/org/apache/openwhisk/http/BasicRasService.scala	16.66% <0.00%> (-83.34%)	:arrow_down:
...scheduler/queue/ElasticSearchDurationChecker.scala	0.00% <0.00%> (-78.85%)	:arrow_down:
...la/org/apache/openwhisk/core/invoker/Invoker.scala	1.58% <0.00%> (-68.26%)	:arrow_down:
...pache/openwhisk/core/invoker/InvokerReactive.scala	0.00% <0.00%> (-60.00%)	:arrow_down:
...apache/openwhisk/core/ack/MessagingActiveAck.scala	0.00% <0.00%> (-54.55%)	:arrow_down:
...whisk/connector/kafka/KafkaMessagingProvider.scala	29.26% <0.00%> (-41.47%)	:arrow_down:
...tainerpool/docker/DockerClientWithFileAccess.scala	56.00% <0.00%> (-36.00%)	:arrow_down:
... and 136 more

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[style95]

@mcdan Would it be ok to merge this PR?