Make use of vulcand dynamic bucket config

I would suggest to simplify the logic. 

I would  rather make it explicit choice for this particular middleware: it would be either a static explicit rate limit, or a header based rate limit. 

Again, if I wanted a safe-failover I have two options:

* Create a two rate limiters - one with dynamic configuration, another with static
* Provided a static reply when I fail to read the value from the database honestly

Otherwise users can be confused (as you see on my example)