Nonce worker: only reveal nonce if cycle position is less than nonce_revelation_threshold

What

Check that nonce_revelation_threshold has not passed before revealing nonce. If the threshold has passed, then the baker cannot reveal any nonces from the previous block so they can be safely dropped.

Why

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR