Puma large file upload demo

This is a simple app to demonstrate memory used by Puma for large file uploads and compare it to proposed changes in PR puma/puma#3062

Steps to reproduce:

• Run the app bundle exec puma -p 3000
• Visit http://localhost:3000/
• Watch memory of ruby process with top, MacOS Activity Monitor, etc
• Use Chrome dev tools to throttle network bandwidth so you can monitor memory usage as upload progresses. I created a "Fast WiFi Upload" profile with 20 MBit/s upload bandwidth. See https://css-tricks.com/throttling-the-network/
• Choose a large video file (100Mb is good). Click "Upload" and watch memory use as the upload progresses. You will noticed process memory increase. I typically see memory grow from ~40MB to ~90MB during the upload.
• Sometimes garbage collection runs, dropping memory usage during the upload. This seems to be common when running several uploads in a row. But memory usage will continue to grow after that.
• Memory will jump when Puma hands the request off to Rack. This can probably also be improved. But that will require a change to Rack and it's out of scope for this PR.

Steps to demonstrate benefits of PR puma/puma#3062

• Update the Gemfile to use the PR version of Puma: gem "puma", github: "willkoehler/puma", branch: "reduce_read_body_memory"
• Restart the app and repeat steps above. You will noticed that memory stays fixed during the upload.

Testing using curl and memory logging from puma_worker_killer

• Run the app with puma_worker_killer: bundle exec puma -p 3000 --config puma.rb -w 1
• Send a POST request with curl: curl --form "video=@your_video.mp4" --limit-rate 10M http://localhost:3000/upload
• Puma will log memory usage in the console