feat(f3): prepare for f3 bootstrap

[Stebalien]

Related Issues

fixes #596

Proposed Changes

This patch:

1. Bootstraps F3 when we hit the F3BootstrapEpoch (when non-negative).
2. Refuses any/all dynamic manifests once we get within one finality of said epoch.
3. Sets the F3 network name for mainnet to "filecoin".

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• [ ] Commits have a clear commit message.
• [ ] PR title conforms with contribution conventions
• [ ] Update CHANGELOG.md or signal that this change does not need it per contribution conventions
• [ ] New features have usage guidelines and / or documentation updates in
  • [ ] Lotus Documentation
  • [ ] Discussion Tutorials
• [ ] Tests exist for new functionality or change in behavior
• [ ] CI is green