* Consistent order fields of Timestamp/BlockID fields in CanonicalVote and
CanonicalProposal
* update spec too
* Introduce and use IsZero & IsComplete:
- update IsZero method according to spec and introduce IsComplete
- use methods in validate basic to validate: proposals come with a
"complete" blockId and votes are either complete or empty
- update spec: BlockID.IsNil() -> BlockID.IsZero() and fix typo
* BlockID comes first
* fix tests