[SPARK-4961] [CORE] Put HadoopRDD.getPartitions forward to reduce DAGScheduler.JobSubmitted processing time

HadoopRDD.getPartitions is lazyied to process in DAGScheduler.JobSubmitted. If inputdir is large, getPartitions may spend much time.
For example, in our cluster, it needs from 0.029s to 766.699s. If one JobSubmitted event is processing, others should wait. Thus, we
want to put HadoopRDD.getPartitions forward to reduce DAGScheduler.JobSubmitted processing time. Then other JobSubmitted event don't
need to wait much time. HadoopRDD object could get its partitons when it is instantiated.
We could analyse and compare the execution time before and after optimization.
TaskScheduler.start execution time: [time1__]
DAGScheduler.JobSubmitted (excluding HadoopRDD.getPartitions or TaskScheduler.start) execution time: [time2_]
HadoopRDD.getPartitions execution time: [time3___]
Stages execution time: [time4_____].
(1) The app has only one job
(a)
The execution time of the job before optimization is [time1__][time2_][time3___][time4_____].
The execution time of the job after optimization is....[time1__][time3___][time2_][time4_____].
In summary, if the app has only one job, the total execution time is same before and after optimization.
(2) The app has 4 jobs
(a) Before optimization,
job1 execution time is [time2_][time3___][time4_____],
job2 execution time is [time2__________][time3___][time4_____],
job3 execution time is................................[time2____][time3___][time4_____],
job4 execution time is................................[time2_____________][time3___][time4_____].
After optimization, 
job1 execution time is [time3___][time2_][time4_____],
job2 execution time is [time3___][time2__][time4_____],
job3 execution time is................................[time3___][time2_][time4_____],
job4 execution time is................................[time3___][time2__][time4_____].
In summary, if the app has multiple jobs, average execution time after optimization is less than before.
Since this `getPartitions` method is guaranteed to only be called once, I think we can just move this logging to its call site in `RDD.scala` (e.g. add a block near where we assign to `partitions_`.