Client provide uncles (#1609)

* feat: add children function to backend

* feat: add test for children hashes

* feat: add uncles function to client

* fix: improve uncles function adds few more tests

* fix: remove children when reverting

* fix: typo and spec version

substrate/core/client/src/leaves.rs

@@ -84,7 +84,6 @@ impl<H, N> LeafSet<H, N> where
 	/// Read the leaf list from the DB, using given prefix for keys.
 	pub fn read_from_db(db: &KeyValueDB, column: Option<u32>, prefix: &[u8]) -> error::Result<Self> {
 		let mut storage = BTreeSet::new();
-
 		for (key, value) in db.iter_from_prefix(column, prefix) {
 			if !key.starts_with(prefix) { break }
 			let raw_hash = &mut &key[prefix.len()..];