fix duplicating&learning(3/n): plog will be lost if decree is step back case when duplicating

[foreverneverer]

Bug Report

for duplicating, if some log hasn't been confirmed, it will be reserve and send the potential secondary, the case which is named step back. for this case, we need resetlog files in /learn as current plog file in /plog, the origin code https://github.com/XiaoMi/rdsn/blob/v2.2/src/replica/replica_learn.cpp#L1526 point this. However, the reset_from is not been implemented correctly, XiaoMi/rdsn#845 is resolve the case.

Version

https://github.com/apache/incubator-pegasus/tree/v2.2.0-RC0