Add locking feature to prevent stale code

Adds locking feature that works with [ember-cli-rails-addon](https://github.com/rondale-sc/ember-cli-rails-addon) (the companion addon in node land)

The `ember-cli-rails-addon` uses the `preBuild` hook to write a file called `preBuild.lock` to the output and the `postBuild` hook to delete that file and write `postBuild.lock`

The middleware we are now injecting blocks until the `preBuild` file is not found and the `postBuild` **is** found before continuing the request.  This results in a block until ember-cli has fully built the output.

Cheers :beer:
I like the configurable build_timeout