Minor Cleanup of StreamGraphGeneator

[Chen Qin]

Hi There,

I am thinking of doing a minor refactor of StreamGraphGenerator.

There are some transforms share same logic that can potentially reusable.
Also, long list of if else seems hard to read, might better off abstract
interface & offer default implementation.

Bonus point would be allow configuration overwrite of this "transformer"
with job level customized implementation.

Any suggestions?

Thanks,
Chen