Refactor State object

The main purpose of this patch is to ensure that implicit invocation only ever happens when the core communicates information for the frontend to optionally consume, but explicit function calls are always used for the frontend to instruct the core to do something.

This was not the case for how we handle task retries.

Orthogonally to this, State object is now refactored to have more consistent API comments and full pep484 type hinting in place, which helps for clarity specifically in the signatures for callbacks passed along to the State object.