Custom LINQ engine

[Turnerj]

This kick starts the work for #87

[codecov[bot]]

Codecov Report

Merging #112 into master will decrease coverage by 4.67%. The diff coverage is 60.64%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   91.74%   87.07%   -4.68%     
==========================================
  Files          71       78       +7     
  Lines        2496     2932     +436     
  Branches      294      348      +54     
==========================================
+ Hits         2290     2553     +263     
- Misses        134      286     +152     
- Partials       72       93      +21

Impacted Files	Coverage Δ
...ure/Mapping/Processors/PropertyMappingProcessor.cs	84.21% <0%> (-5.27%)	:arrow_down:
...e/Linq/Translation/Translators/SelectTranslator.cs	10.71% <10.71%> (ø)
...rastructure/Linq/Translation/DefaultTranslators.cs	100% <100%> (ø)
...re/Linq/Translation/Translators/WhereTranslator.cs	100% <100%> (ø)
.../Linq/Translation/Translators/OrderByTranslator.cs	16.66% <16.66%> (ø)
...rk/Infrastructure/Linq/Translation/StageBuilder.cs	45.45% <45.45%> (ø)
...tructure/Linq/Translation/ExpressionTranslation.cs	66.47% <66.47%> (ø)
...frastructure/Linq/Translation/TranslationHelper.cs	71.42% <71.42%> (ø)
... and 5 more

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[Turnerj]

Of note, attempt to support $lookup through custom LINQ engine. This would effectively add support for basic joins between collections.

[Turnerj]

The process now will be to add the "low hanging fruit" queries like GroupBy etc, then write translators for GeoNear and Text search. Once done, we remove the pre-stage logic as it all will be handled by the new LINQ translator system. The updated GetExecutionModel logic will no longer do any reflection and will retrieve the correct serializer by its own method.

At this point, it is just a matter of weeding out the existing tests to pass, then making sure that each component is adequately tested itself.

Once this is done, the next wave of changes will be writing an optimization layer for queries. This will be done directly with the BsonDocument where it should be easier than with expressions.