Asynchronous increment for memcached

[ElvinEfendi]

Currently when a request is not throttled this middleware adds an extra latency to request processing by the time it takes to increment a counter in memcached (assuming memcached is used as a store).

We can make memcached increments async by batching them locally per worker and then flushing them into memcached in a timer every second or so. This should be properly evaluated though to see how much it would impact the precision of the algorithm if at all.