[SPARK-2251] fix concurrency issues in random sampler

The following code is very likely to throw an exception:

~~~
val rdd = sc.parallelize(0 until 111, 10).sample(false, 0.1)
rdd.zip(rdd).count()
~~~

because the same random number generator is used in compute partitions.
Could dropping this implicit break source and binary compatiblity? I think we'd like to avoid asking people to make code changes to upgrade to a bug-fix release, even if the API's are marked as developer. Can you just leave the existing argument and just ignore it?