Parallel merge sort (#6162)

[tustvold]

Which issue does this PR close?

Closes#6162

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Will run benchmarks prior to merge

Are there any user-facing changes?

[alamb]

I plan to review this carefully tomorrow morning

[alamb]

The code looks great. Thank you @tustvold

I agree some benchmarks are in order but I think the code itself is great.

[alamb]

Nice

[alamb]

This looks beautiful

[tustvold]

It isn't a huge win, but I'll take it

sort utf8 low cardinality
time: [9.0271 ms 9.0396 ms 9.0527 ms]
change: [-11.727% -11.548% -11.370%] (p = 0.00 < 0.05)
Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
3 (3.00%) high mild
1 (1.00%) high severe
merge sorted utf8 high cardinality
time: [7.8055 ms 7.8186 ms 7.8341 ms]
change: [-3.5327% -3.0466% -2.5856%] (p = 0.00 < 0.05)
Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
2 (2.00%) high mild
2 (2.00%) high severe
sort merge utf8 high cardinality
time: [8.0414 ms 8.0505 ms 8.0599 ms]
change: [-2.5665% -2.3550% -2.1393%] (p = 0.00 < 0.05)
Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
2 (2.00%) high mild
sort utf8 high cardinality
time: [12.386 ms 12.413 ms 12.442 ms]
change: [-15.231% -14.371% -13.530%] (p = 0.00 < 0.05)
Performance has improved.
Found 8 outliers among 100 measurements (8.00%)
7 (7.00%) high mild
1 (1.00%) high severe
sort utf8 tuple time: [19.995 ms 20.037 ms 20.082 ms]
change: [-23.549% -23.240% -22.940%] (p = 0.00 < 0.05)
Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
1 (1.00%) high mild
3 (3.00%) high severe
sort mixed dictionary tuple
time: [21.851 ms 21.939 ms 22.033 ms]
change: [-13.963% -13.555% -13.128%] (p = 0.00 < 0.05)
Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
6 (6.00%) high mild
sort mixed tuple time: [19.054 ms 19.176 ms 19.307 ms]
change: [-13.179% -12.578% -11.928%] (p = 0.00 < 0.05)
Performance has improved.
Found 12 outliers among 100 measurements (12.00%)
11 (11.00%) high mild
1 (1.00%) high severe

There is no change the merge benches and the benches for primitive arrays (as they don't use SortPreservingMerge)

[alamb]

I think we should merge it