EBookBackend: Use GThreadPool instead of g_io_scheduler_push_job().

Once the old-style backend API is gone we can port everything to GTask,
but for now be conservative with the changes.
ECalBackend: Use GThreadPool instead of g_io_scheduler_push_job().

Once the old-style backend API is gone we can port everything to GTask,
but for now be conservative with the changes.