Proto: Increase nonce revelation threshold from 32 to 256 blocks

Context

As a requirement for VDF, the current proposal defines a nonce revelation threshold, which is the number of blocks at the beginning of a cycle within which a nonce revelation needs to be included. This MR makes the following changes:

• Increase nonce_revelation_threshold 8x from 32 to 256 blocks. Also increase the VDF difficulty (the time it takes to compute a VDF result and proof) by the same factor.
• Better test for assigning the value of nonce_revelation_threshold when not on mainnet (ensures the invariant nonce_revelation_threshold < blocks_per_cycle is respected)
• Update the Kathmandu doc with the changes in proto_alpha and update the values of the changed constants in both docs.

Manually testing the MR

Reviewers should check the consistency of the 8x increase to the relevant constants in proto_alpha.

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