SPARK-1577: Enabling reference tracking by default in GraphX KryoRegistrator.

[jegonzal]

We had originally disabled reference tracking by default. However,this creates serious issues in the spark-shell. For example the following code will fail:

class A(a: String) extends Serializable
val x = sc.parallelize(Array.fill(10)(new A("hello")))
x.collect

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[rxin]

Is the problem A has a reference to the outer level repl object? Maybe a better solution here is to turn reference tracking off when we serialize closures in the shell mode ... @mateiz thoughts?

[mateiz]

We could do that too, but how would we detect that we're in the shell here? Maybe one way is to set the track references SparkConf field to false and don't override it here if it is set.

[mateiz]

@jegonzal @rxin is this still needed for GraphX to work in the shell in 1.0 or do you guys have a workaround?

[mateiz]

Actually it looks like this will be subsumed by #724. You should close this pull request, as GitHub won't automatically close it.