Refactor Consistency Check

[ajoaoff]

Original issue opened by @cmagnobarbosa at kytos#122.

Currently, the consistency check logic is in the main class of the flow_manager NApp, increasing the size and maintenance cost of this class. Therefore, the consistency check must be refactored and moved.

References

• https://martinfowler.com/articles/class-too-large.html
• https://pypi.org/project/mccabe/