`BlockId` removal: `CallApiAt::state_at` (#13394)

* BlockId removal: CallApiAt::state_at

It changes the arguments of:
- `CallApiAt::state_at`,
- `Client::code_at`,
- `Client::state_at`

from: `BlockId<Block>` to: `Block::Hash`

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: parity-processbot <>