[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**
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**Which service(s) is this request for?**
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**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
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No repository files, tests, or entry points are named; start by checking whether this roadmap repository contains implementation code for Lambda@Edge invocation handling. Done would mean defining and delivering support for multiple concurrent invocations on one instance, including the behavior and constraints needed for the stated authentication and CloudFront use case.

Written by the indexing model from the issue text.