No natural place to flush resources in FileBasedWriter

FileBasedWriter API does not have a natural place to flush resources opened by the writer.

For example, if you create an OutputStream in your Writer using the FileBasedSink's channel and this OutputStream buffers outputs, there is no natural place to call its flush() method.

Maybe something like finishWrite() to match the existing prepareWrite(WritableByteChannel channel) can work.