Clean up storage of an index's datasources

For some reason, while processors just use a single array with enabled processor IDs mapped to their settings, for datasources we have two properties, datasources (array of plugin IDs) and datasource_configs (map of plugin IDs to their settings). I don't think there's any reason for that, is there? I think we should change it to the same as for processors, a single array in datasources mapping enabled plugin IDs to settings (possibly empty).

Also, while we're at it, we might also want to change tracker to a single property, with tracker_id and settings nested (just like currently for the individual processors). That would bring a bit of a system into how we store plugins.

Estimated Value and Story Points

This issue was identified as a Beta Blocker for Drupal 8. We sat down and figured out the value proposition and amount of work (story points) for this issue.

Value and Story points are in the scale of fibonacci. Our minimum is 1, our maximum is 21. The higher, the more value or work a certain issue has.

Value: 1
Story Points: 3