Do not call initialize_block before any runtime api (#8953)

* Do not call `initialize_block` before any runtime api

Before this change we always called `initialize_block` before calling
into the runtime. There was already support with `skip_initialize` to skip
the initialization. Almost no runtime_api requires that
`initialize_block` is called before. Actually this only leads to higher
execution times most of the time, because all runtime modules are
initialized and this is especially expensive when the block contained a
runtime upgrade.

TLDR: Do not call `initialize_block` before calling a runtime api.

* Change `validate_transaction` interface

* Fix rpc test

* Fixes and comments

* Some docs

substrate/bin/node-template/runtime/src/lib.rs

@@ -365,8 +365,9 @@ impl_runtime_apis! {
 		fn validate_transaction(
 			source: TransactionSource,
 			tx: <Block as BlockT>::Extrinsic,
+			block_hash: <Block as BlockT>::Hash,
 		) -> TransactionValidity {
-			Executive::validate_transaction(source, tx)
+			Executive::validate_transaction(source, tx, block_hash)
 		}
 	}