Always use column_index.

[ueshin]

Now that we introduced multi-index columns, we should use column_index rather than data_columns to be consistent access between single and multi index.
We still have a ways to go, so this includes a workaround to work with the data_columns-based approach.
I will address them in the separate PRs.

[codecov-io]

Codecov Report

Merging #648 into master will decrease coverage by 1.89%.
The diff coverage is 97.72%.

@@            Coverage Diff            @@
##           master     #648     +/-   ##
=========================================
- Coverage   93.24%   91.34%   -1.9%     
=========================================
  Files          32       32             
  Lines        5267     5291     +24     
=========================================
- Hits         4911     4833     -78     
- Misses        356      458    +102

Impacted Files	Coverage Δ
databricks/koalas/indexes.py	91.58% <ø> (ø)	⬆️
databricks/koalas/series.py	92.93% <100%> (ø)	⬆️
databricks/koalas/indexing.py	92.85% <100%> (-0.54%)	⬇️
databricks/koalas/utils.py	97.98% <100%> (+0.08%)	⬆️
databricks/koalas/internal.py	95.91% <100%> (-0.75%)	⬇️
databricks/koalas/frame.py	94.46% <92.59%> (-0.12%)	⬇️
databricks/koalas/usage_logging/__init__.py	23.14% <0%> (-74.08%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50% <0%> (-50%)	⬇️
databricks/koalas/__init__.py	76.92% <0%> (-7.7%)	⬇️
databricks/conftest.py	93.02% <0%> (-4.66%)	⬇️
... and 2 more

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