Handling delayed anchor block commitments

Issue #65 proposes a mechanism for ensuring that invalid blocks are not created through L1 forks, but it exacerbates a potential issue for hyperchains: delayed block commits. If the L2 miners submit a block commit intended for block 1 in the underlying chain, but it isn’t incorporated until block 2, the commitment will be lost, and the L2 miners would need to issue a new block commitment. This will cause a drop in throughput.

Instead, the hyperchains contract could allow for backdated commits to proceed successfully. If the block commit method accepts both a `parent_index_hash` *and* a block height, then the hyperchains contract can use `at-block` to verify that the `parent_index_hash` matches the would-have-been parent at that block height, and validate the transaction.