trivial: Remove the unused pk_backend_reset_job() vfunc
trivial: Clarify what pk_backend_job_reset() actually does
dummy: Fix make check by not calling pk_backend_job_finished() in the threaded code
trivial: Remove the incorrectly-used pk_backend_reset_job()

Now the job lifecycle is tied to the transaction this becomes much simpler.
trivial: Ignore invalid TransactionDB entries
Allow pk_backend_job_set_allow_cancel() after ErrorCode has been set