Implement testing for Token Bucket algorithm

[shalarewicz]

Use a TDD approach to verify correctness of the Token Bucket algorithm used for rate limiting incoming GraphQL requests.

Testing should not be concerned with computing an incoming requests complexity. Each request should be considered to consume 1 token.

Testing should verify the following;

1. Requests are limited as expected based on user IP address.
2. CAPACITY and REFILL_RATE are accepted input parameters.
3. Size of the token bucket never exceeds the set CAPACITY
4. Token bucket is refilled at the provided REFILL_RATE