When FileName changes, we should reset FileNumber back to 0 instead of continuing where we left off.

But it might be somewhat tricky - e.g. OnChange fires in the middle of execution of WriteMessagesAsync, causing the FileNumber to go back to zero. If this happens while we're scanning through some previous files to find out if they exist, it could cause us to suffix the first file with the new name with something nonzero. I'll file a follow-up issue for this one, for now I think it's fine if the numbering for the new file names pick up where the old ones left off.