Make storage obligation update during conract renewal atomic

MERGE REQUEST

MR Guidelines

Overview

Previously when renewing contracts, updating the storage obligation of the renewed contract and creating the storage obligation for the new contract wouldn't happen atomically on disk. This MR changes that so that we either update both or neither.

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods or updated methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to workflow
• Issue added to Sia-UI repo for new supporting features
• Changelog File Created