Introduce prefixed storage with enumeration (#4185)

* Introduce storage_next allowing iteration.  (without childtries)

* Implement prefixed storage

* impl cache in client_storage_cache (needs test)

* switch overlay change to btreemap

* Revert "impl cache in client_storage_cache"

This reverts commit c91a4848916eba87184b3dc4722cea81aec9339d.

the storage cache cannot be used this way

* Revert "Implement prefixed storage"

This reverts commit 4931088126a427082d7310ed7e83b8eea966bc20.

* Impl StoragePrefixedMap for all map storages

* remove comment

* Move all overlays to BTreeMap

* btreemap iteration improvment

* impl for child tries

* impl tests for childs

* fix

* remove cache comment

* Fix grumble

substrate/client/db/src/lib.rs

@@ -151,6 +151,14 @@ impl<B: BlockT> StateBackend<Blake2Hasher> for RefTrackingState<B> {
 		self.state.exists_child_storage(storage_key, key)
 	}
 
+	fn next_storage_key(&self, key: &[u8]) -> Result<Option<Vec<u8>>, Self::Error> {
+		self.state.next_storage_key(key)
+	}
+
+	fn next_child_storage_key(&self, storage_key: &[u8], key: &[u8]) -> Result<Option<Vec<u8>>, Self::Error> {
+		self.state.next_child_storage_key(storage_key, key)
+	}
+
 	fn for_keys_with_prefix<F: FnMut(&[u8])>(&self, prefix: &[u8], f: F) {
 		self.state.for_keys_with_prefix(prefix, f)
 	}

substrate/client/db/src/storage_cache.rs

@@ -544,6 +544,14 @@ impl<H: Hasher, S: StateBackend<H>, B: BlockT> StateBackend<H> for CachingState<
 		self.state.exists_child_storage(storage_key, key)
 	}
 
+	fn next_storage_key(&self, key: &[u8]) -> Result<Option<Vec<u8>>, Self::Error> {
+		self.state.next_storage_key(key)
+	}
+
+	fn next_child_storage_key(&self, storage_key: &[u8], key: &[u8]) -> Result<Option<Vec<u8>>, Self::Error> {
+		self.state.next_child_storage_key(storage_key, key)
+	}
+
 	fn for_keys_in_child_storage<F: FnMut(&[u8])>(&self, storage_key: &[u8], f: F) {
 		self.state.for_keys_in_child_storage(storage_key, f)
 	}