handle default type in proto value

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.