Async getUpstream

[JonathanHopeDMRC]

Prerequisites

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🚀 Feature Proposal

The getUpstream callback can be sync or async.

Motivation

We have a (probably unusual) need for getUpstream to support async. We have a sophisticated tenancy strategy that requires us to reach out to dbs/caches/etc to resolve the correct upstream.

Example

reply.from(/some/path', {
  getUpstream: async () => { return await someAsyncFunc() }
}

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.

Ideally we should not be paying a perf penalty when getUpstream() is sync, so some callback/promises gymnastics would be needed.