Store build steps and logs as structured data [breaking change]

[bradrydzewski]

This pull request is a work in progress and will allow us to store the individual build steps and logs as structured data in the database, per issue #1857. The first commit adds the necessary data structures. Subsequent commits will begin altering the guts of drone to use these data structures.

This will be a substantial change to the codebase. Expected completion in ~7 days.

• add new database tables and queries
• update agent to use new pipeline rpc
• update rpc to save step logs
• update rpc to save step status
• update rpc to update build once steps complete
• update rpc to handle zombie steps
• update event bus to handle step events
• update ui to handle step events
• update ui to fetch logs per-step on expansion
• fix restart and deploy, which is now completely broken
• confirm matrix works
• confirm cancel works