Add a Smooth Rate Limiter

[robrua]

Currently Orianna only offers a bursty rate limiter for use with the Riot API. For large keys, it can sometimes be preferable to make requests in a "smooth" way, limiting bursting to reduce the likelihood of bad responses from the Riot API when it is under heavy load.

Such a rate limiter should be thread-safe and meet (or supplement) the existing rate limiter interface.