Re-institute async mode

[joshleblanc]

This is less of a "this is ready to be merged", and more of an effort to reinvigorate conversation around this feature. I cobbled this together as a proof of concept because I simply couldn't use the multi-process approach on the server I'm trying to deploy to. This is related to #343, and tries to naively reimplement this

I originally stumbled on this feature because I was testing Kamal on a 1GB VPS. The initial deploy would work, however there wasn't enough resource headroom for the second deploy. I narrowed this down to SQ using ~100MB per process, essentially taking up half the available memory on the system.

After implementing this change, I saw the application memory drop from ~700MB to ~270MB.

The usage is simply placing this in puma.rb

if ENV["SOLID_QUEUE_IN_PUMA"]
  plugin :solid_queue
  solid_queue_mode :async
end

I currently have this deployed to a 1GB droplet - the application is running quickly, background jobs are running, and redeploys are succeeding