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/api/src/backend.rs

@@ -420,11 +420,6 @@ pub trait Backend<Block: BlockT>: AuxStore + Send + Sync {
 	/// Returns state backend with post-state of given block.
 	fn state_at(&self, block: BlockId<Block>) -> sp_blockchain::Result<Self::State>;
 
-	/// Destroy state and save any useful data, such as cache.
-	fn destroy_state(&self, _state: Self::State) -> sp_blockchain::Result<()> {
-		Ok(())
-	}
-
 	/// Attempts to revert the chain by `n` blocks. If `revert_finalized` is set
 	/// it will attempt to revert past any finalized block, this is unsafe and
 	/// can potentially leave the node in an inconsistent state.