Resolve "Panic when loading contract after clean shutdown of siad"

MERGE REQUEST

Overview

This MR fixes a bug that was caused by a limit on the header size of contracts. This requires the following steps.

• Move from a single .contract file to a .header and .roots file
• Add test to confirm we can now store contracts with large headers
• Make managedInsertContract ACID
• Add compat code to split up existing contracts
• Add testing for the compat code

Example for Visual Changes

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

Closes #3950 (closed)