Make contract confirmation index NOT NULL

[chris124567]

Unlike with proofs or resolutions, a contract is guaranteed to be confirmed once it is in the database. Previously this data was stored as nullable to make it convenient to update alongside resolution index / resolution transaction ID in updateFileContractIndices/updateV2FileContractIndices. We also split the the chain index into separate fields, i.e.

-- excerpted from `last_contract_revision` / `v2_last_contract_revision`
    confirmation_height BLOB NOT NULL,
    confirmation_block_id BLOB NOT NULL REFERENCES blocks(id) ON DELETE CASCADE,

So that we can have a direct reference to the block.

Note: Base of this branch is v2 to avoid complicated rebasing once that's merged but it's not really a v2 feature so I don't think it needs to be merged into there.