Support retrieving/expanding on all node predicates

algo/uidlist.go

cmd/dgraph/main.go

cmd/dgraph/main_test.go

gql/parser.go

gql/parser_test.go

posting/list.go

query/outputnode.go

query/query.go

query/query_test.go

rdf/parse.go

worker/backup.go

worker/task.go

x/values.go

x/values_test.go