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/test-utils/runtime/transaction-pool/src/lib.rs

@@ -326,14 +326,9 @@ impl sc_transaction_pool::ChainApi for TestApi {
 
 	fn block_header(
 		&self,
-		at: &BlockId<Self::Block>,
+		hash: <Self::Block as BlockT>::Hash,
 	) -> Result<Option<<Self::Block as BlockT>::Header>, Self::Error> {
-		Ok(match at {
-			BlockId::Number(num) =>
-				self.chain.read().block_by_number.get(num).map(|b| b[0].0.header().clone()),
-			BlockId::Hash(hash) =>
-				self.chain.read().block_by_hash.get(hash).map(|b| b.header().clone()),
-		})
+		Ok(self.chain.read().block_by_hash.get(&hash).map(|b| b.header().clone()))
 	}
 
 	fn tree_route(