[SPARK-7855] Move bypassMergeSort-handling from ExternalSorter to own component

Spark's `ExternalSorter` writes shuffle output files during sort-based shuffle. Sort-shuffle contains a configuration, `spark.shuffle.sort.bypassMergeThreshold`, which causes ExternalSorter to skip sorting and merging and simply write separate files per partition, which are then concatenated together to form the final map output file.

The code paths used during this bypass are almost completely separate from ExternalSorter's other code paths, so refactoring them into a separate file can significantly simplify the code.

In addition to re-arranging code, this patch deletes a bunch of dead code.  The main entry point into ExternalSorter is `insertAll()` and in SPARK-4479 / #3422 this method was modified to completely bypass in-memory buffering of records when `bypassMergeSort` takes effect. As a result, some of the spilling and merging code paths will no longer be called when `bypassMergeSort` is used, so we should be able to safely remove that code.

There's an open JIRA ([SPARK-6026](https://issues.apache.org/jira/browse/SPARK-6026)) for removing the `bypassMergeThreshold` parameter and code paths; I have not done that here, but the changes in this patch will make removing that parameter significantly easier if we ever decide to do that.

This patch also makes several improvements to shuffle-related tests and adds more defensive checks to certain shuffle classes:

- DiskBlockObjectWriter now throws an exception if `fileSegment()` is called before `commitAndClose()` has been called.
- DiskBlockObjectWriter's close methods are now idempotent, so calling any of the close methods twice in a row will no longer result in incorrect shuffle write metrics changes.  Calling `revertPartialWritesAndClose()` on a closed DiskBlockObjectWriter now has no effect (before, it might mess up the metrics).
- The end-to-end shuffle record count metrics tests have been moved from InputOutputMetricsSuite to ShuffleSuite.  This means that these tests will now be run against all shuffle implementations rather than just the default shuffle configuration.
- The end-to-end metrics tests now include a test of a job which performs aggregation in the shuffle.
- Our tests now check that `shuffleBytesWritten == totalShuffleBytesRead`.
- FileSegment now throws IllegalArgumentException if it is constructed with a negative length or offset.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #6397 from JoshRosen/external-sorter-bypass-cleanup and squashes the following commits:

bf3f3f6 [Josh Rosen] Merge remote-tracking branch 'origin/master' into external-sorter-bypass-cleanup
8b216c4 [Josh Rosen] Guard against negative offsets and lengths in FileSegment
03f35a4 [Josh Rosen] Minor fix to cleanup logic.
b5cc35b [Josh Rosen] Move shuffle metrics tests to ShuffleSuite.
8b8fb9e [Josh Rosen] Add more tests + defensive programming to DiskBlockObjectWriter.
16564eb [Josh Rosen] Guard against calling fileSegment() before commitAndClose() has been called.
96811b4 [Josh Rosen] Remove confusing taskMetrics.shuffleWriteMetrics() optional call
8522b6a [Josh Rosen] Do not perform a map-side sort unless we're also doing map-side aggregation
08e40f3 [Josh Rosen] Remove excessively clever (and wrong) implementation of newBuffer()
d7f9938 [Josh Rosen] Add missing overrides; fix compilation
71d76ff [Josh Rosen] Update Javadoc
bf0d98f [Josh Rosen] Add comment to clarify confusing factory code
5197f73 [Josh Rosen] Add missing private[this]
30ef2c8 [Josh Rosen] Convert BypassMergeSortShuffleWriter to Java
bc1a820 [Josh Rosen] Fix bug when aggregator is used but map-side combine is disabled
0d3dcc0 [Josh Rosen] Remove unnecessary overloaded methods
25b964f [Josh Rosen] Rename SortShuffleSorter to SortShuffleFileWriter
0d9848c [Josh Rosen] Make it more clear that curWriteMetrics is now only used for spill metrics
7af7aea [Josh Rosen] Combine spill() and spillToMergeableFile()
6320112 [Josh Rosen] Add missing negation in deletion success check.
d267e0d [Josh Rosen] Fix style issue
7f15f7b [Josh Rosen] Back out extra cleanup-handling code, since this is already covered in stop()
25aa3bd [Josh Rosen] Make sure to delete outputFile after errors.
931ca68 [Josh Rosen] Refactor tests.
6a35716 [Josh Rosen] Refactor logic for deciding when to bypass
4b03539 [Josh Rosen] Move conf prior to first use
1265b25 [Josh Rosen] Fix some style errors and comments.
02355ef [Josh Rosen] More simplification
d4cb536 [Josh Rosen] Delete more unused code
bb96678 [Josh Rosen] Add missing interface file
b6cc1eb [Josh Rosen] Realize that bypass never buffers; proceed to delete tons of code
6185ee2 [Josh Rosen] WIP towards moving bypass code into own file.
8d0678c [Josh Rosen] Move diskBytesSpilled getter next to variable
19bccd6 [Josh Rosen] Remove duplicated buffer creation code.
18959bb [Josh Rosen] Move comparator methods closer together.

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