Implement auto-update software for Agora

[bpalaggi]

As a node operator, I need a software that keeps an automatically updated agora node on my server in order to have seamless integration with the chain.

Definition of Done:

• ~[ ] The auto-updater is separate from Agora~
• [ ] The auto-updater is the default package pointed to on the website
• ~[ ] The auto-updater can never be updated~
• [ ] The auto-updater never fails
• [ ] The auto-updater can rollback a failing upgrade
• [ ] The auto-updater uses multiple mirrors
• [ ] The auto-updater supports server-side downtime