Migrate all evaluator API to async

[jondequinor]

Describe the bug Ensemble Evaluator has a lot of synchronous API. It is very IO-centric, so it should be pure asyncio. It used to interface with a lot of sync legacy code, and I think there was some uneasiness on how to deal with it, so a lot of it was made sync.

Expected behavior Developing evaluator stuff should be an exercise in calling sync stuff from async, not calling sync stuff calling async stuff calling sync stuff.

See https://github.com/equinor/ert/issues/2844 and, you know, the code…