Add a database

[jonasmalacofilho]

This would serve many purposes:

• restart work, specially notifications
• shard the build load across servers
• provide some ui to view progress and past results
• rebuild pull requests when their base branch is updated

[jonasmalacofilho]

Idea: use GitHub commit statuses as a key value database. Basically, it might be possible to implement a database of sorts while keeping the servers stateless, by keeping all state in GitHub.

(summarized from #23)