The previous implementation used int64 for both, which can be mixed up
easily. Using distinct types adds a layer of type safety checked by the
compiler.
* Refactor SourceManager to remove Enrollment
Initializing the Source will be the responsibility of the caller. The
SourceManager exposes a GetIDs method for getting a source and job ID.
* Update tests
* Update engine usage
* Update apiClient interface to have one GetIDs method
* Update SourceManager usage in engine
* 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
