feat(rate-limiting): leaky bucket rate limiter

[vartec]

Leaky bucket rate limiter which allows for defining both max burst rate as well as sustained rate limit.

• While the bucket is not yet full, the requests are accepted until it's full (bucket size = max burst limit)
• When the bucker is full, new request are accepted only after a drop drips out (drip rate = sustained rate limit)

This is a metered (i.e. counter) implementation, which is far less resource heavy, but only limits the traffic, doesn't attempt to smooth it out.