4.x: example to show rate limiting using a semaphore

[barchetta]

Add an SE example that shows the following:

1. Use of WebServers max-concurrent-requests to put a hard limit on number of concurrent requests.
2. Use of a Sempahore in business logic to implement a simple rate limiter as described in Use Semaphores to Limit Concurrency.

The use case we want to show is gracefully handling a burst of requests while also providing a hard limit.