Support entitysets with cycles in their relationship graph

For example, a self-loop:
```
Entities:
employees
Relationships:
employees.manager_id -> employees.id
```
Or, a cycle involving multiple entities:
```
Entities:
users
roles
Relationships
users.role_id -> roles.id
roles.creator_id -> users.id
```
We should be able to create features that traverse these cycles once or more. For example, "The average salary of the direct reports of an employee's direct reports": `MEAN(employees.employees.salary)`.

To support this there are at least two places in the code which assume there are no cycles and so will need to be updated:

1. `EntitySet.has_unique_forward_path`: When searching for paths this ignores entities which have already been seen – only traversing cycles once. In the case of the "employees" entityset above it would say that there is a unique path from `employees` to `employees`, even though there are infinite.
2. `DeepFeatureSynthesis.build_features`: Currently this will get stuck in an infinite loop when run on an entityset with a cycle (in the call to `EntitySet.get_backward_entities(eid, deep=True)`). To fix this we could add a `max_relationship_depth` param to limit the number of relationships which will be traversed. This would change existing behavior even in entitysets without cycles because there is currently no such limit (`max_depth` only limits the nesting of features, not the lengths of their relationship paths).