LinkedQL + Bolt: .out().is() is slower than .has().union(.has())

**Description**
When executing:
```javascript
g.V().out(predicate).is(a, b)
```
It is signifcantly slower than
```javascript
g.V().has(predicate, a).union(g.V().has(predicate, .b)
```
I presume it has to do with the way we use indexes for `.out().is()` which is not optimized like `.has()`