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/frame/merkle-mountain-range/primitives/src/lib.rs

@@ -406,7 +406,6 @@ sp_api::decl_runtime_apis! {
 	/// API to interact with MMR pallet.
 	pub trait MmrApi<Hash: codec::Codec> {
 		/// Generate MMR proof for a leaf under given index.
-		#[skip_initialize_block]
 		fn generate_proof(leaf_index: u64) -> Result<(EncodableOpaqueLeaf, Proof<Hash>), Error>;
 
 		/// Verify MMR proof against on-chain MMR.
@@ -414,7 +413,6 @@ sp_api::decl_runtime_apis! {
 		/// Note this function will use on-chain MMR root hash and check if the proof
 		/// matches the hash.
 		/// See [Self::verify_proof_stateless] for a stateless verifier.
-		#[skip_initialize_block]
 		fn verify_proof(leaf: EncodableOpaqueLeaf, proof: Proof<Hash>) -> Result<(), Error>;
 
 		/// Verify MMR proof against given root hash.
@@ -423,7 +421,6 @@ sp_api::decl_runtime_apis! {
 		/// proof is verified against given MMR root hash.
 		///
 		/// The leaf data is expected to be encoded in it's compact form.
-		#[skip_initialize_block]
 		fn verify_proof_stateless(root: Hash, leaf: EncodableOpaqueLeaf, proof: Proof<Hash>)
 			-> Result<(), Error>;
 	}