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/consensus/aura/src/lib.rs

@@ -957,7 +957,7 @@ mod tests {
 			compatibility_mode: Default::default(),
 		};
 
-		let head = client.header(&BlockId::Number(0)).unwrap().unwrap();
+		let head = client.expect_header(client.info().genesis_hash).unwrap();
 
 		let res = worker
 			.on_slot(SlotInfo {
@@ -972,6 +972,6 @@ mod tests {
 			.unwrap();
 
 		// The returned block should be imported and we should be able to get its header by now.
-		assert!(client.header(&BlockId::Hash(res.block.hash())).unwrap().is_some());
+		assert!(client.header(res.block.hash()).unwrap().is_some());
 	}
 }