[SPARK-14724] Use radix sort for shuffles and sort operator when possible

## What changes were proposed in this pull request?

Spark currently uses TimSort for all in-memory sorts, including sorts done for shuffle. One low-hanging fruit is to use radix sort when possible (e.g. sorting by integer keys). This PR adds a radix sort implementation to the unsafe sort package and switches shuffles and sorts to use it when possible.

The current implementation does not have special support for null values, so we cannot radix-sort `LongType`. I will address this in a follow-up PR.

## How was this patch tested?

Unit tests, enabling radix sort on existing tests. Microbenchmark results:

```
Running benchmark: radix sort 25000000
Java HotSpot(TM) 64-Bit Server VM 1.8.0_66-b17 on Linux 3.13.0-44-generic
Intel(R) Core(TM) i7-4600U CPU  2.10GHz

radix sort 25000000:                Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
-------------------------------------------------------------------------------------------
reference TimSort key prefix array     15546 / 15859          1.6         621.9       1.0X
reference Arrays.sort                    2416 / 2446         10.3          96.6       6.4X
radix sort one byte                       133 /  137        188.4           5.3     117.2X
radix sort two bytes                      255 /  258         98.2          10.2      61.1X
radix sort eight bytes                    991 /  997         25.2          39.6      15.7X
radix sort key prefix array              1540 / 1563         16.2          61.6      10.1X
```

I also ran a mix of the supported TPCDS queries and compared TimSort vs RadixSort metrics. The overall benchmark ran ~10% faster with radix sort on. In the breakdown below, the radix-enabled sort phases averaged about 20x faster than TimSort, however sorting is only a small fraction of the overall runtime. About half of the TPCDS queries were able to take advantage of radix sort.

```
TPCDS on master: 2499s real time, 8185s executor
    - 1171s in TimSort, avg 267 MB/s
(note the /s accounting is weird here since dataSize counts the record sizes too)

TPCDS with radix enabled: 2294s real time, 7391s executor
    - 596s in TimSort, avg 254 MB/s
    - 26s in radix sort, avg 4.2 GB/s
```

cc davies rxin

Author: Eric Liang <ekl@databricks.com>

Closes #12490 from ericl/sort-benchmark.

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