[Code health] MutationRepository Cleanup

[scolsen]

I think our overall approach and design will be simplified if we dumb down the logic the MutationRepository offers and offload some processing into methods defined directly on our mutation value types.

[gino-m]

Working on #2867 now; exposing more fine-grained logic from MutationRepository has a few benefits:

• It makes the workers more readable, since a named handle for these methods are available rather than needing to depend on the details of how states are represented internally.
• It makes code less fragile by keeping the moving the "how" states are represented closer to the data rather than the business logic.

Closing for now, @scolsen please reopen if you have strong feelings about this!