flush() should be called for every append on block scan verification log

RollingLogsImpl#out is PrintWriter and it have the default buffersize of 8kB.

So until the 8kB buffer is filled all scan verification entries will not be flushed, hence there will be chance loosing this scan information if the datanode gets restarted. And One more scan will happen for these blocks.

8kB will have ~80 entries, so 80 blocks needs to be re-scanned again.