FileLogger: Fix compile against Hadoop trunk

In Hadoop trunk (currently Hadoop 3.0.0), the deprecated
FSDataOutputStream#sync() method has been removed.  Instead, the
FSDataOutputStream#hflush method fills the same role.  We should call
hflush if it is available.  This patch uses reflection to maintain
support for old versions of Hadoop that do not have hflush, but which do
have the deprecated sync method.