[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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