MultiSourceTap hashcode 

Adding a simpler hashcode for ScaldingMultiSourceTap. Cascading's hashcode implementation can significantly slow down flow planning when there are a large number of underlying taps (>500):
https://github.com/Cascading/cascading/blob/2.6/cascading-core/src/main/java/cascading/tap/MultiSourceTap.java#L229

Not very familiar with hashcode implementations in general. Is there a better way to do this while still keeping it independent of the number of underlying taps? Thanks.