added mongodb collection as client for supporting rate limiting using mongodb

[MrSpark2591]

Hi,

I have added Mongo db support for rate limiter.

This one was much needed feature as you can see multiple request has been raised in the issues section to support mongodb as storage backend.

We are already using it in our product and will be backing it and we will also provide all the support needed for the same.

if you need more info please let me know :)

[MrSpark2591]

@ded any progress on this ?