Use _final instead of _flush

[Tapppi]

Now that node 8 and readable-stream include the _final method, we should use that when closing down instead of _flush to make sure all processing is done before 'finish' event fires.

The only change should be that the Writable side of the transform does not close after everything has been passed to transform function but after everything has been processed, as should happen according to node docs.

Fixes #4

[jomaora]

Any news about this fix?

[Tapppi]

@jomaora waiting for feedback from @mafintosh

[piglovesyou]

I respectfully merged this into my public package @piglovesyou/parallel-transform. I put some tests and it works very well even in require('stream').pipeline, emitting callback after all buffers are consumed. I appreciate your work, @Tapppi .