Close iterators (#1542)

* Add Close() function to all the iterators

* Add defer iterator.Close() whenever we open an iterator

* Add isClosed to all iterators and panic/return error if used after closing

Co-authored-by: Svarog <feanorr@gmail.com>

infrastructure/db/database/ldb/cursor.go

@@ -106,7 +106,8 @@ func (c *LevelDBCursor) Close() error {
 		return errors.New("cannot close an already closed cursor")
 	}
 	c.isClosed = true
-
 	c.ldbIterator.Release()
+	c.ldbIterator = nil
+	c.bucket = nil
 	return nil
 }