avoid rehashing blocks

[evanmcc]

this is a draft PR for being more thrifty about doing work on whole blocks. it's only one approach, and it's possible that it isn't the right one.

another approach would be to immediately denormalize every block accepted into various individually fetchable fields (bonus points for separate, iterable transactions with types outside of the serialization) and then simply mark them as valid and present separately, doing away with the whole distinction between valid and plausible blocks.