Fix siac stop instant shutdown

MERGE REQUEST

MR Guidelines

Overview

This MR fixes siac stop seemingly killing siad right away. The problem is that when using siac stop, the main thread won't wait for the shutdown to complete. The endpoint also caused a deadlock which made it even worse due to the web server shutting down itself. That's why closing the server was moved to a separate goroutine within the API.

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