Make links unique with the help of indices

[kirillvasilenko]

The links duplication problem is solved now in the following way:

1. On adding a link, we check if there is already the same link. If there is, we just silently skip new one, if there is no, we add the new one.
2. We do this checking only if there is an appropriate index for the edge class (for the in and out edges). If there is no an index, we do not check for link duplicates.

Check out the new tests to see how it works.

[andrii0lomakin]

Hi @kirillvasilenko
Please modify the findEdge method.
It should issue an SQL query to find edges with in-out parameters equal to those provided.

[kirillvasilenko]

@andrii0lomakin , done.