Utilise async request-reply pattern for long-running requests

[kimburgess]

Is your feature request related to a problem? Please describe.
A number of endpoints exist which expose operations that, depending on the environment, may result in long-running synchronous requests. An example of this is the backfill endpoint or driver compilation.

Describe the solution you'd like
Utilise an asynchronous request-reply pattern.

Describe alternatives you've considered
Continue to provide synchronous responses, but clearly document warnings around this.