Accept queries spanning multiple facts with common dimensions

Changes include : 

* Adds CandidateFact sets in CubeQueryContext, which cover all the measures covered.
* Modifies LighestFact and LeastPartition resolvers to work on candidate sets instead of individual fact.
* Adds MultiFactHQLContext to construct the multi fact query. Refactored earlier HQLContext into DimOnlyHQLContext and SingleFactHQLContext. Created abstract classes which can be used all DimOnly/SingleFact/MultiFact.
* Updated JoinResolver to prune paths of single fact when join clauses are picked.
* CandidateFact carries the ASTs corresponding to the fields selected by the fact.


Can be done. But leaving HQLContextInterface class as is, because there can other implementations of HQLContextInterface which provides query strings, though not there right now. 

Removed the class HQLContext and added a method in CubeQueryContext to create HQLContextInterface, as nothing else was done in this class.