Refactored test code
Clean up of cardinality estimation

* Mostly test clean up around AssumeIndependenceQueryGraphCardinalityModelTest
* Also tweaking of the selectivity of relationship uniqueness predicates
Handling of var length in the independence cardinality model
Merge pull request #3513 from systay/varlength-cardinality

Var-length cardinality estimation
Cleaned up ExecutionContext
Produce RowSpec when creating Pipes
Turned ExecutionContext into a trait

MapExecutionContext is the main implementation of ExecutionContext
Stable WIP. All green except varlength pipe building
NodeByIdSeek
Rewrite expressions to use speced contexts