Use resolvers context to access dgraph and postgres

[charlie-wasp]

Resolve #101

It may be a good idea to use shared context in resolvers to obtain connections to our storages (dgraph and postgres). At least, this is how it said in Apollo server docs 😁

@gzigzigzeo was concerned about resources freeing and connection pooling. I guess, pg-promise got us covered here, it has a connection pool, it's described in the README. Dgraph case is more complicated for me because its JS client uses protobuf (so we're talking about gRPC connections, right?), and I am not familiar with that. However, they are just an HTTP connections? What problems can we have here? What bothers me is that dgraph-js docs state, that resources must be cleared explicitly. We do that on ingest, but we don't do that on queries. Maybe we can use express server, and put dgraph "resources freeing" in some "after request" hook?

What do you think about it?

Yeah, also I renamed vaguely named Connection class here to Dgraph

---

This change is