Split Skynet code into own files.

MERGE REQUEST

Overview

No functional changes. Move Skynet code into its own files in order to keep the Renter files a little shorter.

Example for Visual changes (ie Screenshot)

N/A

Issues Closed

Closes #3880 (closed)

Checklist

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

• All new methods, or updating 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 work flow
• Issue added to Sia-UI repo for new supporting features
• Changelog updated