Improve performance by using Elastic when searching for relations (when we can)

## Problem to Solve

We have some use case today where it should be possible to resolve a list of relations directly in Elastic instead of Grakn in order to improve the performance.

## Current Workaround

Nothing

## Proposed Solution

Base on previous work of indexing, refactor the function paginateRelationships of grakn.js to use elastic when we can
