Enable workhorse queue limitation for the API

With the following values:

gitlab_workhorse['api_limit'] = 8
gitlab_workhorse['api_queue_limit'] = 100
gitlab_workhorse['api_queue_duration'] = 60

We will need to watch for file descriptors as we are going to leave those requests hanging for a while.

We should be monitoring the number of requests that result in 503 for timeout exceeded and 429 for queue overflow.