Disallow vars in facets filter.
Adds fail-test for facets at root.
Adds failure tests.

Fix for loop go routine's bug for variable capturing.
use dst instead of sg

Add failing tests

Returning error in rdf/parsing if int does not fit in int32.
Renames fns in facet_types.go
Takes care of strings starting with ints and has alphabets.

gql/parse @facets and @facets(facetkey1, facetkey2).
Adds AddMapChild in outputNode.
Facets output structure @facets : { <predicate-name> : { <facet-key> : <facet-val>...}, ..more-predicates.. }
Update edges when facets change and edges get deleted.

rdf/facets parsing : <subject> <predicate> <object> <label>? '(' facetkey=facetVal (, facetKey=facetVal)* ')' .
Type deduction of facetVal supported. Bool, Int, Float, String and DateTime are allowed FacetVal.
This is part of adding facets (edge key val) support in Dgraph.

Using faster IntersectWith in place of IntersectSorted

Need to remove dependency on intermediate nodes for cases which are
better solved using properties on edges.