Remove Backend::destroy_state (#5068)

* Remove `Backend::destroy_state`

This removes the `destroy_state` function of `Backend` and instead moves
the functionality into the `Drop` implementation of the state. This
makes it much easier to work with the state, as the user no longer needs
to call` destroy_state` manually. However, it requires that we switch
from `RwLock` to `ReentrantMutex` as while importing a block we maybe
need to lock again in `drop`.

* Bring back the `RwLock` and some other clean ups

* Fix compilation

substrate/client/block-builder/src/lib.rs

@@ -224,17 +224,11 @@ where
 			&state,
 			changes_trie_state.as_ref(),
 			parent_hash,
-		);
-
-		// We need to destroy the state, before we check if `storage_changes` is `Ok(_)`
-		{
-			let _lock = self.backend.get_import_lock().read();
-			self.backend.destroy_state(state)?;
-		}
+		)?;
 
 		Ok(BuiltBlock {
 			block: <Block as BlockT>::new(header, self.extrinsics),
-			storage_changes: storage_changes?,
+			storage_changes,
 			proof,
 		})
 	}