* Add Display method to SourceUnit and Kind member to the CommonSourceUnit
* Make SourceUnitID return the ID and a kind
These two values together uniquely represent a unit.
* Refactor UnitHook to block the scan if finished metrics aren't handled
* Log once when back-pressure is detected
* Add hook channel size metric
* Use plural "metrics" for consistency
* Replace LRU cache with map
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.
* 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
* Support fatal errors in job reports
* WIP: JobReporter and JobInspector
* WIP: JobReportHook and JobReportRef
* Add ChunkError type and asyncRun helper method
* Rename JobReport to JobProgress
* Return a closed channel from Done when the JobProgress is nil
* Comment catchFirstFatal function
