restore task can't be stoped manually

1、pegasus don‘t provide the tool to stop the “restore_app” task manualy.
2、when try stop it by “drop {app}”，the task still run，but the “query_restore_status” show “restore failed, because some partition's data is damaged on cold backup media”.
3、when restore anther app，the task must wait for the last task to complete，otherwise you'll see that progress is zero.

The issue names the restore_app task, the drop {app} command, and query_restore_status; start by reproducing the stop and subsequent-restore sequences described there. Done means a restore task can be stopped manually, its status is accurate, and restoring another app does not wait on the prior task or show zero progress.

Written by the indexing model from the issue text.