Re #7822. Keep the last log entries instead of clearing.

The previous way presented problems with unchanging or
slowly varying logs as a chunk could be left with an empty
log that the analysis relied on having.
The particular problem one was 'EnergyRequest', which was
only appearing in the first chunk.
I think that we can count on the logs coming in in time order, so the
fact that the underlying methods don't check or sort should not be a
problem.

Code/Mantid/Framework/LiveData/src/SNSLiveEventDataListener.cpp

@@ -1239,8 +1239,8 @@ namespace LiveData
     //Copy geometry over.
     API::WorkspaceFactory::Instance().initializeFromParent(m_eventBuffer, temp, false);
 
-    // Clear out the old logs
-    temp->mutableRun().clearTimeSeriesLogs();
+    // Clear out the old logs, except for the most recent entry
+    temp->mutableRun().clearOutdatedTimeSeriesLogValues();
 
     // Lock the mutex and swap the workspaces
     {