server: force watchmand to run as a separate process

I am not married to this proposal. For now, I am looking for Concept ACK/Concept NACK first. Please, don't spend time doing a detailed review (yet).

The key observation is that we only run captaind and watchmand as separate processes. We never run captaind with an embedded watchmand.

However, we do have quite some code to set it up and test it. In many cases we are testing the embedded version. Testing the embedded version in CI and running the 2-process version in prod feels like a recipe for disaster.

This MR kills the feature of embedding watchmand inside captaind.

• check or remove this line after you've added a CHANGELOG entry or if your PR doesn't need one (they should go into CHANGELOG/unreleased/)