Improvements to no-op subsegment behavior

[willarmiros]

Description of changes:
This PR includes a few changes involving no-op subsegment behavior to correct regressions:

1. Add a new shouldPropagate field to subsegments, so that we can distinguish between no-op subsegments that are unsampled and no-op subsegments which are created from context missing exceptions.
2. In LambdaSegmentContext, fixed the endSubsegment behavior to use the context missing strategy instead of always throwing an exception.
3. Changed the logging logic for ending no-op subsegments to make debug logs more clear

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #294 (a4c8a35) into master (297a0bf) will increase coverage by 0.07%.
The diff coverage is 57.69%.

@@             Coverage Diff              @@
##             master     #294      +/-   ##
============================================
+ Coverage     58.86%   58.93%   +0.07%     
- Complexity     1202     1205       +3     
============================================
  Files           131      131              
  Lines          5061     5070       +9     
  Branches        590      593       +3     
============================================
+ Hits           2979     2988       +9     
+ Misses         1807     1806       -1     
- Partials        275      276       +1     

Impacted Files	Coverage Δ
...va/com/amazonaws/xray/handlers/TracingHandler.java	38.13% <0.00%> (-0.39%)	⬇️
.../main/java/com/amazonaws/xray/AWSXRayRecorder.java	63.74% <0.00%> (ø)
.../amazonaws/xray/contexts/LambdaSegmentContext.java	78.57% <0.00%> (-2.32%)	⬇️
...a/com/amazonaws/xray/entities/DummySubsegment.java	0.00% <0.00%> (ø)
...ray/javax/servlet/AWSXRayServletAsyncListener.java	61.11% <ø> (-7.08%)	⬇️
...onaws/xray/contexts/ThreadLocalSegmentContext.java	77.77% <25.00%> (-3.62%)	⬇️
...aws/xray/proxies/apache/http/TracedHttpClient.java	40.44% <80.00%> (-0.46%)	⬇️
...mazonaws/xray/interceptors/TracingInterceptor.java	60.00% <100.00%> (+0.88%)	⬆️
...va/com/amazonaws/xray/entities/NoOpSubSegment.java	93.90% <100.00%> (+0.31%)	⬆️
...n/java/com/amazonaws/xray/entities/Subsegment.java	100.00% <100.00%> (ø)
... and 3 more

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[willarmiros]

@anuraaga any chance for a second look here and on #293?