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/primitives/session/src/lib.rs

@@ -110,7 +110,7 @@ impl GetValidatorCount for MembershipProof {
 #[cfg(feature = "std")]
 pub fn generate_initial_session_keys<Block, T>(
 	client: std::sync::Arc<T>,
-	at: &BlockId<Block>,
+	at: Block::Hash,
 	seeds: Vec<String>,
 ) -> Result<(), sp_api::ApiError>
 where
@@ -125,7 +125,7 @@ where
 	let runtime_api = client.runtime_api();
 
 	for seed in seeds {
-		runtime_api.generate_session_keys(at, Some(seed.as_bytes().to_vec()))?;
+		runtime_api.generate_session_keys(&BlockId::Hash(at), Some(seed.as_bytes().to_vec()))?;
 	}
 
 	Ok(())