BlockId removal: refactor: HeaderBackend::header (#12874)

* BlockId removal: refactor: HeaderBackend::header

It changes the arguments of:
- `HeaderBackend::header`,
- `Client::header`,
- `PeersClient::header`
- `ChainApi::block_header`

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

This PR is part of BlockId::Number refactoring analysis (paritytech/substrate#11292)

* non-trivial usages of haeder(block_id) refactored

This may required introduction of dedicated function:
header_for_block_num

* fmt

* fix

* doc fixed

* ".git/.scripts/fmt.sh"

* BlockId removal: refactor: HeaderBackend::expect_header

It changes the arguments of `HeaderBackend::expect_header` method from: `BlockId<Block>` to: `Block::Hash`

* ".git/.scripts/fmt.sh"

* readme updated

* ".git/.scripts/fmt.sh"

* fix

Co-authored-by: parity-processbot <>

substrate/client/basic-authorship/README.md

@@ -8,7 +8,7 @@ let mut proposer_factory = ProposerFactory::new(client.clone(), txpool.clone(),
 
 // From this factory, we create a `Proposer`.
 let proposer = proposer_factory.init(
-	&client.header(&BlockId::number(0)).unwrap().unwrap(),
+	&client.header(client.chain_info().genesis_hash).unwrap().unwrap(),
 );
 
 // The proposer is created asynchronously.