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/primitives/offchain/src/lib.rs

@@ -28,12 +28,10 @@ sp_api::decl_runtime_apis! {
 	#[api_version(2)]
 	pub trait OffchainWorkerApi {
 		/// Starts the off-chain task for given block number.
-		#[skip_initialize_block]
 		#[changed_in(2)]
 		fn offchain_worker(number: sp_runtime::traits::NumberFor<Block>);
 
 		/// Starts the off-chain task for given block header.
-		#[skip_initialize_block]
 		fn offchain_worker(header: &Block::Header);
 	}
 }