fix(dgraph): Do not return reverse edges from expandEdges

[jarifibrahim]

We don't need to return reverse edges when we expand the types.
When alpha is running in strict mutations mode and if we try to
delete a node with a type that contains a reverse edge then that
could cause the deletion to fail because the type would be expanded
with the reverse edges.

With this PR, we don't process the reverse edges when processing a type.
Fixes DGRAPH-3057

---

This change is 

[manishrjain]

Also get someone to create tests for this change. We need two. 1. We shouldn't accept reverse edge mutations. and 2. We should not auto-expand a reverse edge if type has it.

Reviewable status: 0 of 2 files reviewed, all discussions resolved (waiting on @pawanrawal and @vvbalaji-dgraph)

[danielmai]

Does this change apply only to set/delete mutations or from queries using expand(_all_) as well?

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @pawanrawal and @vvbalaji-dgraph)

[jarifibrahim]

Does this change apply only to set/delete mutations or from queries using expand(all) as well?

We removed the restriction we had earlier. This PR now ony removes the reverse edges while expanding edges (this works with expand(all). I verified this along with @pawanrawal )

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @pawanrawal and @vvbalaji-dgraph)