Add a config to perform natural ordering when head.

[ueshin]

Koalas doesn't guarantee the row ordering, so head could return some rows from distributed partitions and the result is not deterministic, which might confuse users. (See #411, #1064, or #1219)

>>> import databricks.koalas as ks
>>> spark = ks.utils.default_session({'spark.master': 'local-cluster[4, 2, 1024]'})
>>> spark.conf.set('spark.sql.execution.arrow.enabled', True)
>>> kdf = ks.DataFrame({'a': range(10)})
>>> pdf = kdf.to_pandas().head(3)

>>> for _ in range(100):
...     assert kdf.head(3).to_pandas().equals(pdf), 'NOT equals.'
... else:
...     print('equals.')
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
AssertionError: NOT equals.

If compute.ordered_head is set to True, Koalas performs natural ordering beforehand, but it will cause a performance overhead.

>>> ks.options.compute.ordered_head = True
>>>
>>> for _ in range(100):
...     assert kdf.head(3).to_pandas().equals(pdf), 'NOT equals.'
... else:
...     print('equals.')
...
equals.

I don't think we should enable it by default due to the overhead, but it will help users debug.

[codecov-io]

Codecov Report

Merging #1231 into master will increase coverage by 0.25%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #1231      +/-   ##
==========================================
+ Coverage   95.18%   95.43%   +0.25%     
==========================================
  Files          35       35              
  Lines        7265     7740     +475     
==========================================
+ Hits         6915     7387     +472     
- Misses        350      353       +3

Impacted Files	Coverage Δ
databricks/koalas/config.py	98.98% <ø> (ø)	⬆️
databricks/koalas/frame.py	97.48% <80%> (+0.51%)	⬆️

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[HyukjinKwon]

Yeah, I think adding this configuration wouldn't hurt.

[HyukjinKwon]

I think it's worth enough to try.

[ueshin]

Thanks! I'd merge this for now. Please feel free to leave comments if any.