fix. raft follower will rollback itself when it misses a certain log

if a leader fails to update its last_log_id_sent (maybe caused be
follower fail or network issue), it will then try to send some
duplicated logs in the next appendLog request.

```
                      commitId_: 99
                          |
                          v
local wal: |-------------------|
req.append_entries:        |----------------|
```

in this case, follower will do rollback even if the logs are the same.

src/kvstore/raftex/RaftPart.cpp

@@ -1731,8 +1731,10 @@ void RaftPart::processAppendLogRequest(const cpp2::AppendLogRequest& req,
       TermID lastTerm = (numLogs == 0) ? req.get_last_log_term_sent()
                                        : req.get_log_str_list().back().get_log_term();
       auto localWalIt = wal_->iterator(firstId, lastId);
+      bool hasConflict = false;
       for (size_t i = 0; i < numLogs && localWalIt->valid(); ++i, ++(*localWalIt), ++diffIndex) {
         if (localWalIt->logTerm() != req.get_log_str_list()[i].get_log_term()) {
+          hasConflict = true;
           break;
         }
       }
@@ -1749,7 +1751,9 @@ void RaftPart::processAppendLogRequest(const cpp2::AppendLogRequest& req,
 
       // Found a difference at log of (firstId + diffIndex), all logs from (firstId + diffIndex)
       // could be truncated
-      wal_->rollbackToLog(firstId + diffIndex - 1);
+      if (hasConflict) {
+        wal_->rollbackToLog(firstId + diffIndex - 1);
+      }
       firstId = firstId + diffIndex;
       numLogs = numLogs - diffIndex;
     }