Flush buffer again after first "flush" event

[compulim]

In file transport, before the FileStream is opened, logs are buffered to _buffer.

Once the FileStream is opened, file transport will flush _buffer to disk and wait for the first drain event from the stream. After the drain event is received, _buffer will be decommissioned and subsequent write will direct to the FileStream itself.

There are race condition when the logs are written inbetween the flush call and the drain event. Logs written inbetween them will still be stored in _buffer (because self.opening is still true). Because we did not flush the _buffer again after drain, some logs could be orphaned there forever.

The fix is to flush the _buffer again after the drain event is received.

[compulim]

Will include it in another pull request.