[issue] Potential posiblity that drop the msg and not process when a system failure occurs

[Acaishiba]

we should have a backup mechanism that makes sure every message will be processed no matter what accident happen.

• check mechanism, check if every mag has been recorded into intent pool
• Checker, the relayer that is not on duty, this will ensure a lower burden when the relayer is offline
• buffer Pool, record the unexpectedly dropped message into pool, executed by the Checker