* Add SourceManager to Engine struct
* Update Engine methods to use the SourceManager
* Fix GCS test
The original was testing that `Init()` errors weren't surfaced in
`Finish()`, but the `SourceManager` changed that behavior.
* JobProgress race fixes
* Add contextual values
* Remove unused code
* Add debug logs
* Rename WithConcurrency to WithConcurrentSources
* Always forward chunks to the output chunks channel