feat(Query): Allow filters in expand(_all_) queries on predicates pointing to nodes

[all-seeing-code]

Motivation

Enable queries that can declare filters on predicates while using expand(_all_) directive.
Example:

q(func: eq(name, "Michonne")) {
	expand(_all_)
	friend @filter(eq(alive, true)){
		expand(_all_)
	}
}

Note: Queries that explicitly ask for scalar predicates while using expand(_all_) will continue to throw.

This PR fixes: DGRAPH-2524

Further discussion: https://discuss.dgraph.io/t/expand-with-predicate-filters/10611

Components affected by this PR

Query Package particularly queries involving expand(_all_) directive.

Does this PR break backwards compatibility?

No

Testing

Added following tests in:

1. TestFilterAtSameLevelOnUIDWithExpand

Fixes

Fixes DGRAPH-2524

---

This change is 

[pawanrawal]

Reviewed 1 of 1 files at r1, 2 of 2 files at r2.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @anurags92)

---

query/query.go, line 400 at r2 (raw file):

		return false
	}
	// TODO(Anurag): Below check can be expanded to check deeper equivalence

Instead of a TODO, just mention what are the limitations of the current approach.

---

query/query4_test.go, line 628 at r2 (raw file):

func TestFilterAtSameLevelOnUIDWithExpand(t *testing.T) {
	query := `{
		q(func: eq(name, "Michonne")) {

Can you try this as well?

michone {
  expand(_all_) {
    expand(_all_)
  }
  friend @filter(...) {
    expand(_all_)
  }
}

michonne {
  expand(_all_) {
    friend @filter(eq(alive, true)) {
      name
    }
  }
}

---

query/query4_test.go, line 637 at r2 (raw file):

	js := processQueryNoErr(t, query)
	// res, _ := json.MarshalIndent(js, " ", "\t")
	fmt.Println("JS: ", string(js))

should be removed