This enables limiting concurrency. It is a naive approach which
will reject requests as soon as they exceed the maximum number
of in-flight requests.

It is a port of the following PR from the new watchdog code:
https://github.com/openfaas-incubator/of-watchdog/pull/54



Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>