[NFC] delete filenetworktask safer

and eliminate some annoying warnings
QCoreApplication::postEvent: Unexpected null receiver

reasoning: QScopePointer use delete operator to delete the resource directly
when it is destoryed. this will force it to use deleteLater method instead.
Merge pull request #332 from haiwen/delete_scopepointer_safer

[NFC] delete filenetworktask safer