Policy to rate-limit action executions

According to https://docs.stackstorm.com/reference/policies.html, we have concurrency and retry policies.

And while concurrency is useful, it's not enough in some cases.

This example comes as a note from SREcon 17 Dublin where we configured demo: Twitter sensor to listen on all #srecon tweets and change LED colors on every tweet.
> Click here for photo
The problem is that sometimes there were too many tweets/per second and we definitely didn't want to change the LED colors with such a high rate. Here limiting executions/per time period could be very useful.

The proposal is to implement rate-limiting policy so we could limit the action executions based on time executions/second, executions/time interval and cancel everything that exceeded the rate limit window.

Proposed policy example

---
name: rate-limit-http-downloads
description: > Run max 3 'core.http' actions/second
               and cancel everything that exceeds the limit
enabled: true
resource_ref: core.http
# alternative: 
# policy_type: action.limit
policy_type: action.rate_limit
parameters:
    # maximum number of executions per time interval window
    threshold: 3
    # 100ms, 1s, 315s, 15m, 1h
    interval: 1s
    # cancel actions exceeding the 3 actions/s limit
    action: cancel

We'll need additionally to filter/limit the executions by input arguments, same as https://docs.stackstorm.com/reference/policies.html#action-concurrency-attr

I believe this is typical and pragmatic instrument that might be very popular in a wild.

Start with the policy reference at docs.stackstorm.com/reference/policies.html and compare the existing concurrency and retry policy entry points. Define how threshold, interval, cancellation, and input-argument filtering should behave, then identify the relevant policy tests before implementing. Done means the rate-limit example and its boundary cases are covered by tests and documented.