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/transaction-pool/src/graph/pool.rs

@@ -96,7 +96,7 @@ pub trait ChainApi: Send + Sync {
 	/// Returns a block header given the block id.
 	fn block_header(
 		&self,
-		at: &BlockId<Self::Block>,
+		at: <Self::Block as BlockT>::Hash,
 	) -> Result<Option<<Self::Block as BlockT>::Header>, Self::Error>;
 
 	/// Compute a tree-route between two blocks. See [`TreeRoute`] for more details.