[SPARK-4452][Core]Shuffle data structures can starve others on the same thread for memory

[lianhuiwang]

Currently when there are 2 spillable objects in a thread, ShuffleMemoryManager does not work well. some time latter spillable just can ask 5MB.
when join on #5868#, because join has ExternalAppendOnlyMap and UnsafeShuffleExternalSorter’s page size is 128MB, UnsafeShuffleExternalSorter always cannot ask PAGE_SIZE memory and throw IOException("Unable to acquire " + PAGE_SIZE + " bytes of memory”).
in the future, when using #6444’s binary processing sort on unsafe-shuffle-sort, the problem is more serious.
in this PR, when previous spillable is finished,add it to ShuffleMemoryManager’s reserved list. when latter spillable ask memory,but ShuffleMemoryManager has no enough memory, it spill previous spillable and get its memory. that make latter spoilable get more memory.
@JoshRosen@andrewor14@sryza i think you are interesting in this. and can you take a look? thanks.

[SparkQA]

Test build #36152 has finished for PR 7130 at commit 5cc88ab.

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[squito]

nit: scala imports before spark imports

[SparkQA]

Test build #36155 has finished for PR 7130 at commit 5280ef0.

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[squito]

nit: spill2 is unused

[SparkQA]

Test build #36157 has finished for PR 7130 at commit c17c047.

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[SparkQA]

Test build #36162 has finished for PR 7130 at commit 2e0b237.

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[SparkQA]

Test build #36171 has finished for PR 7130 at commit 83040cf.

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Test build #36270 has finished for PR 7130 at commit f4737bb.

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[SparkQA]

Test build #36281 has finished for PR 7130 at commit 907366b.

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[JoshRosen]

/bump @andrewor14@rxin, I think that we should look at this before 1.5. I share @lianhuiwang's concerns that our new usages of ShuffleMemoryManager will make the longstanding starvation problem even worse.

@andrewor14, I think that you know a bit more of the history involved in this logic. Didn't we used to have the opposite problem where multiple collections in the same thread (or task, after #7734) may lead to endless spilling behavior?