Bug: Mempool Expiration Nil Pointer

[andy-shi88]

Description

A nil pointer occured everytime the mempool expiration job got some transaction to clean up.

Steps to reproduce

1. add expired mempool
2. run the app, every 5 minutes, clean up should be executed, it'll shutdown.

Expected behavior

Mempool expiration should be ran without errors,

[andy-shi88]

found cause: in expiring the mempool, we used the ExecuteTransaction which require db transaction to be initiated, and it has not.

[andy-shi88]

solution: call BeginTx before begin to undo the transaction.