Release v1.27.0

Changed

• A job counts each step's stop at most once per execution, and remembers which stops it counted in its stored state, so a repeated stop report is never counted twice, even after the job moved to another silo. Custom job storage providers must persist the new StoppedStepsCounted property of JobState.
• Calling Remove() on a job that is already being removed answers RemoveJobError.Unknown when finishing the removal fails, instead of RemoveJobError.JobIsAlreadyBeingRemoved, so the caller knows to try again.
• OnStopped can run more than once for a removal whose first attempt to finish failed.

Fixed

• Removing a job no longer leaves it in Removing forever when its last step's stop report fails. The step keeps reporting its stop, with backoff, until the job has stored it, and a removal that failed to finish (OnStopped, clearing the job, or the final write) is finished again by the next report. A stop from an earlier execution of the step now counts while the job is being removed. (#95)