Remove database, provide Persistor at Start

[djaglowski]

This PR prepares the library for the use of the storage extension framework.

The idea is that state will be persisted at a higher level, and only a simple persistence interface is necessary within this library.

• Removes the concept of a database, and the associated dependency on bbolt
• Reworks the Persister interface, removing Sync and Load, but adding context.Context to all methods
• Passes a Persister to each Start method (Agent.Start, Pipeline.Start, Operator.Start)
• Moves Persister into the operator package, because it is similar to BuildContext