Future major v6 notes

[animir]

1. blockDuration option should be undefined by default, so 0 can be used to block keys forever.
2. Make sure RateLimiterQueue is created with default maxQueueSize even if it isn't set in options object, e.g. new RateLimiterQueue(memoryLimiter, {}) shouldn't result to maxQueueSize set to undefined.
3. Support for points set to 0 and never limit in this case. It is useful for atomic counters.

[daniel-white]

consider adjusting the cb of the different implementations to return asynchronously if tableCreated: true - its not consistent w/ the other times cb is called.

another better design would be to have private constructors and a factory method that returns a promise