log file fix

Note that Interlocked.Increment(ref JobLogBatchCounter) is performed right after the blob save. Because, if the blob save fails for some reason, LastFileName would not be pointing to non-existent file
Also, note that using an int helps us perform thread-safe easily using methods like Interlocked.Increment
So, instead of using a filename we can consider using an int. Then, the code can be something like the following

Interlocked.Exchange(ref LastFileNameCounter, Interlocked.Increment(ref JobLogBatchCounter))