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/lib.rs

@@ -556,7 +556,7 @@ async fn prune_known_txs_for_block<Block: BlockT, Api: graph::ChainApi<Block = B
 
 	log::trace!(target: "txpool", "Pruning transactions: {:?}", hashes);
 
-	let header = match api.block_header(&BlockId::Hash(block_hash)) {
+	let header = match api.block_header(block_hash) {
 		Ok(Some(h)) => h,
 		Ok(None) => {
 			log::debug!(target: "txpool", "Could not find header for {:?}.", block_hash);