[SPARK-2387][Core]Remove Stage's barrier

[lianhuiwang]

based on #1328.
when one task of parent stage is not finished, so other executors is idle. we can pre-start the reduce stage to make good use of these idle executors.
This can achieve better resource utilization and improve the overall job performance, especially when there're lots of executors granted to the application.in my no-cache application's test, it improves the job by about 10%.
@lirui-intel @sryza @rxin

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Test build #23785 has started for PR 3430 at commit ac0a8a1.

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Test build #23786 has finished for PR 3430 at commit cb77455.

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[JoshRosen]

This seems like a complicated change for only a 10% performance improvement, especially when there's probably lower-hanging fruit that's easier / safer to optimize.

[lianhuiwang]

@JoshRosen i think when we need this feature, we just set spark.scheduler.removeStageBarrier=true and then own this performance improvement. it is important that will not affect other's features.

[pwendell]

I'd propose to close this issue. It's a fairly large change and needs more discussion before being seriously considered to be part of spark.