Rename package api to dgraph

Why is the package for api.proto `api`? Shouldn't it be specific to dgraph, maybe `dgraph_api` or simply `dgraph`?

Using `api` gives bit nonsensical usage like here, from grpc generated code.
https://github.com/Swoorup/dgraph-client-rs/blob/master/examples/dgraph_tuto01/main.rs#L40