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/authority-discovery/src/worker/tests.rs

@@ -58,7 +58,7 @@ impl ProvideRuntimeApi<Block> for TestApi {
 impl<Block: BlockT> HeaderBackend<Block> for TestApi {
 	fn header(
 		&self,
-		_id: BlockId<Block>,
+		_hash: Block::Hash,
 	) -> std::result::Result<Option<Block::Header>, sp_blockchain::Error> {
 		Ok(None)
 	}