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.

Finish all pending client reqs, then stopping all nodes, internal grpc conn and syncing watermarks.
Also add shutdown os signal handler.

List of changes:
 - Language tag parsing in queries
 - Implementation of parsing of language lists in format pred@lang1:lang2
 - Improved error handling (related to parsing '@'), covered with tests
 - Lang field in Posting, DirectedEdge and NQuad
 - Support for storing multiple values in posting list

We now give error to the user if there are any invalid fn/arg names
in query : rather than giving random results.