[service] [request] support parallel invocations on a single instance

One of our highly used Node.js lambdas is mostly IO-bound, waiting for a HTTP request to complete. It would be nice if a single lambda instance can handle multiple invocations in parallel, so we can save on start-up cost, have some shared cache and not run into our quota limit as fast (we're capped at 2500 at the moment, but it's not enough).

We'd use a regular Node.js service for this, but in this case we're using Lambda@edge so we're in a bind.

The specific lambda as a auth/acl check in cloudfront before we hit a S3 origin.

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**Tell us about your request**
What do you want us to build?

**Which service(s) is this request for?**
This could be AWS Lambda itself, or integrations with other services.

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

**Are you currently working around this issue?**
How are you currently solving this problem?

**Additional context**
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No se nombran archivos del repositorio, pruebas ni puntos de entrada; empieza comprobando si este repositorio de roadmap contiene código de implementación para gestionar invocaciones de Lambda@Edge. Se consideraría completado definir y entregar compatibilidad con múltiples invocaciones simultáneas en una instancia, incluido el comportamiento y las restricciones necesarios para el caso de uso de autenticación y CloudFront indicado.

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