Cleanup batch_verify_seal and tipset state execution

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Cleaned up types and removed TODO for Switch batch verify seals to use async tasks instead of threads #676 since Rayon will be our best bet here
  • Looked pretty in depth with this, and handling manually or with an async alternative is not as performant or just unnecessarily unergonomic with no benefit in practice
• Wrapped state execution in a spawn_blocking to avoid starving the runtime executor
  • I want to do a larger refactor later of the state manager/cs to follow a similar pattern for the function signatures (&Arc<Self> when the function can possibly spawn a new thread which uses self and &self otherwise, which has no implications outside of this aside from maybe tests and allows us to use and implement caching and other logic on top of them)

Reference issue to close (if applicable)

Closes #676

Other information and links