[hoanle0126]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Description

Fixes #408

• Supports both synchronous and asynchronous (Promise / thenable) getUpstream(request, base) callbacks.
• Preserves the synchronous fast path (typeof dest?.then === 'function') so synchronous getUpstream callbacks incur no Promise allocation overhead.
• Forwards rejected promises from async getUpstream callbacks to onError.