Allow limiting hook concurrency.

[wrouesnel]

When trigger bash scripts or other functions which have some associated runtime, there are many situations where it's desirable to only allow exactly 1 instance of a hook to be executing at a time (or a limited number).

While this could be accomplished with various locking systems at the hook level, these have the downside that they still lead to spawning new shell processes, which are much more expensive then the goroutines which handle the incoming webhooks.

It would be extremely useful if a max-concurrency flag could be specified for a webhook definition which would cause webhook to queue requests that exceed it

[adnanh]

I'd be more than glad to accept a good PR for this feature! 👍

[adnanh]

Future looks bright for this one :-) Just a little bit more polish and it'll be ready!