Addresses: * https://github.com/filecoin-project/lotus/pull/3781/files/a307e4593a005888395db34e5af4691ee50d765d#r491966115 * https://github.com/filecoin-project/lotus/pull/3781/files/a307e4593a005888395db34e5af4691ee50d765d#r491966634 Note: This puts everything into a policy package to avoid a dependency cycle between the build package, the miner package, and the types package. This is also why I introduced a GetPreCommitChallengeDelay function and removed the variable.