[TINKERPOP-2128] Added ExplainStrategy

[vkagamlyk]

Added ExplainStrategy which allows to get traversal explanation for remote traversals and GLV's.

https://issues.apache.org/jira/browse/TINKERPOP-2128

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (a9ff025) 75.20% compared to head (dd275b2) 75.19%.

Files	Patch %	Lines
...lin/language/grammar/TraversalStrategyVisitor.java	50.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@              Coverage Diff              @@
##             3.6-dev    #2367      +/-   ##
=============================================
- Coverage      75.20%   75.19%   -0.01%     
+ Complexity     12322    12321       -1     
=============================================
  Files           1057     1058       +1     
  Lines          63450    63463      +13     
  Branches        6935     6938       +3     
=============================================
+ Hits           47716    47721       +5     
- Misses         13169    13171       +2     
- Partials        2565     2571       +6     

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[vkagamlyk]

Let's return to this topic in the context of TinkerPop 4