Add client options for configuring a connection pool

[mcrumm]

As we did for Cloud Pub/Sub (plataformatec/broadway_cloud_pub_sub#37), should we look to implement pipeline-specific connection pools for BroadwaySQS?

I was about to start on a PR for this when I realized it might be a little more involved than the Pub/Sub change, as we don't have a hard requirement on :hackney here. Should we add one?

[msaraiva]

Not sure. I remember we've discussed some time ago that since broadway producers already work as a pool, the best approach would be to use something like Mint. I'm sceptical that adding these options now would bring any real benefit but I might be wrong. We would need some real benchmark to make sure :)

[josevalim]

So my recommendation is to use github.com/keathley/finch which is still a pool but without the extra copying. :)