Action Level Concurrency Limit

As per our conversations in [Slack](https://openwhisk-team.slack.com/archives/C3TPCAQG1/p1539369187000100) I'm opening an issue to track this idea.

_As a developer I want to set a concurrency limit on a particular function, in order to limit its access to a downstream resource that can't handle too many concurrent requests_

> This is not the same as https://github.com/apache/incubator-openwhisk/pull/3687 , which allows 1 action to concurrently process multiple activations.

Start by reviewing the Slack discussion linked in the issue and compare its proposal with pull request 3687, since the issue says this request is different. Define how a per-action concurrency limit should behave and how it should be verified, including that concurrent access to a downstream resource remains bounded.

Written by the indexing model from the issue text.