consensus::State (de)serialization regressions

[tarcieri]

There are a couple problems with the serialization (or rather, deserialization) implemented in #676 which prevents existing consensus state JSON files from being read. (Note: we can support a new serialization, it's just the deserialization that needs to be backwards compatible)

null block_id field

tendermint_proto::serializers::optional does not appear to handle the field being explicitly null, e.g.:

{"height":"3820001","round":"0","step":2,"block_id":null}

This results in the following error:

invalid type: null, expected struct BlockId at line 1 column 50

total field of parts::Header

This was previously serialized as a string. It now appears to be serialized as an integer. This causes the following error when deserializing an existing file:

 invalid type: string "1", expected u32 at line 1 column 146

[greg-szabo]

The total field was changed in Tendermint Go too. I can write a compatibility layer to be able to deserialize from string OR int, but we should keep in mind that serialization will follow the new normal.

This means that KMS can upgrade from an older version but cannot downgrade after a new version was executed.

[tarcieri]

@greg-szabo that sounds fine. worst case, people can hand-edit the files or blow them away