Pin states in memory so that they are not pruned away while still referenced (#2761)

* State pinning in client

* Canonicalization queue

* Fixed prioritization queue

* possible fix of "hash mismatch"

* Check for pinned discarded states

* Release state before finalization

* Style

* Style

substrate/core/state-machine/src/trie_backend.rs

@@ -179,7 +179,7 @@ impl<S: TrieBackendStorage<H>, H: Hasher> Backend<H> for TrieBackend<S, H> where
 		(root, is_default, write_overlay)
 	}
 
-	fn try_into_trie_backend(self) -> Option<TrieBackend<Self::TrieBackendStorage, H>> {
+	fn as_trie_backend(&mut self) -> Option<&TrieBackend<Self::TrieBackendStorage, H>> {
 		Some(self)
 	}
 }