[Issue 731] Prevent sent pendingItem from being resend again after the reconnection of producer

[WinChua]

Fixes #731

Motivation

As the #731 has show, some msg maybe sent after the reconnection of the producer, no mater the msg has been sent or not, which will cause that the same msg will be sent and consumed mulit time at the application view.

Modifications

A sent flag was added to pendingItem to indicated that the corresponding buffer has been sent successfully. And in the resend logic the sent item will not be sent again.