Optimize window deletion

[gwaramadze]

Optimizations around deleting windows that eliminate the need to deserialize the window key before deletion.

1. WindowedRocksDBPartitionTransaction.delete_window has been removed because it depended on deserialized input parameters. Deleting windows will now be handled by a lower-level PartitionTransaction._delete_bytes method.
2. WindowedRocksDBPartitionTransaction.delete_windows internally will not use get_windows but a lower-level _get_items method which returns serialized keys.
3. WindowedRocksDBPartitionTransaction.expire_windows no longer handles deleting windows. It has been ultimately delegated to the outer scope, specifically the process_window method.