Block status

[lrettig]

The Block data structure doesn't currently have a status field. From the perspective of a node a block can be PENDING (i.e., syntactically valid but contextual status unclear, waiting for hare and tortoise to confirm), VALID/CONFIRMED, or INVALID (i.e., contextually invalid, due to hare and/or tortoise). We should add this, so that if a block later gets invalidated, downstream clients are aware of this and know to drop the transactions it includes.