Fetch runtime code from storage cache when using proofing backend (#9611)

Before we fetched the runtime code from the `TrieBackend` and this lead
to not using the storage cache. Thus, we recalculated the storage hash
for the runtime code on every call into the runtime and this killed the
performance on parachains block authoring. The solution is to fetch the
runtime code from the storage cache, to make sure we use the cached
storage cache.

substrate/client/db/src/storage_cache.rs

@@ -703,7 +703,7 @@ impl<S: StateBackend<HashFor<B>>, B: BlockT> StateBackend<HashFor<B>> for Cachin
 		self.state.child_keys(child_info, prefix)
 	}
 
-	fn as_trie_backend(&mut self) -> Option<&TrieBackend<Self::TrieBackendStorage, HashFor<B>>> {
+	fn as_trie_backend(&self) -> Option<&TrieBackend<Self::TrieBackendStorage, HashFor<B>>> {
 		self.state.as_trie_backend()
 	}
 
@@ -901,9 +901,9 @@ impl<S: StateBackend<HashFor<B>>, B: BlockT> StateBackend<HashFor<B>>
 		self.caching_state().child_keys(child_info, prefix)
 	}
 
-	fn as_trie_backend(&mut self) -> Option<&TrieBackend<Self::TrieBackendStorage, HashFor<B>>> {
+	fn as_trie_backend(&self) -> Option<&TrieBackend<Self::TrieBackendStorage, HashFor<B>>> {
 		self.caching_state
-			.as_mut()
+			.as_ref()
 			.expect("`caching_state` is valid for the lifetime of the object; qed")
 			.as_trie_backend()
 	}