Deprecate the dramatiq_group_callback_barrier_ttl environment variable

[mikeroll]

I don't think this needs to exist. GroupCallbacks is not in the default middlewares and has to be added and configured explicitly anyway, might as well override barrier_ttl too if desired.

Also added GroupCallbacks to the reference while I was at it.

[LincolnPuzey]

Hi @mikeroll, is there a specific problem this is causing? If we are going to remove the env var, I think a deprecation period would be best.

[mikeroll]

Not really a specific problem, I just find the current set of env vars somewhat confusing (as in, why exactly is this an env var and not a class/function/cli argument?). This is one of the more obvious cases IMO.

I would add a deprecation warning if you agree.

[LincolnPuzey]

Yes they are a bit ad-hoc.
I think most control worker behavior, which is not a class/function user code typically calls, so the other option for them would be a cli flag.
Given most are perhaps 'advanced customization', I don't mind them being env vars.

But also yes dramatiq_group_callback_barrier_ttl is an exception to that, where a middleware kwarg would be better.

So lets remove it, but not until 3.0.
For now, add the middleware kwarg, and deprecate the env var.
Thanks!