[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.

Author: Xiangrui Meng <meng@databricks.com>

Closes #1229 from mengxr/fix-sample and squashes the following commits:

f1ee3d7 [Xiangrui Meng] fix concurrency issues in random sampler