Implement set_names() for Index/MultiIndex

[itholic]

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Index.set_names.html#pandas.Index.set_names

>>> idx = ks.Index([1, 2, 3, 4])
>>> idx
Int64Index([1, 2, 3, 4], dtype='int64')

>>> idx.set_names('quarter')
Int64Index([1, 2, 3, 4], dtype='int64', name='quarter')

For MultiIndex

>>> idx = ks.MultiIndex.from_tuples([('a', 'x'), ('b', 'y')])
>>> idx
MultiIndex([('a', 'x'),
            ('b', 'y')],
           )

>>> idx.set_names(['kind', 'year'], inplace=True)
>>> idx
MultiIndex([('a', 'x'),
            ('b', 'y')],
           names=['kind', 'year'])

>>> idx.set_names('species', level=0)
MultiIndex([('a', 'x'),
            ('b', 'y')],
           names=['species', 'year'])

[itholic]

this PR is including fix of rename(),

make inplace=True to return nothing but just change its name like pandas.

existing

>>> idx = ks.Index([1, 2, 3, 4])
>>> idx
Int64Index([1, 2, 3, 4], dtype='int64')

>>> idx.rename('quarter', inplace=True)
Int64Index([1, 2, 3, 4], dtype='int64', name='quarter')

fixed (same with pandas)

>>> idx = ks.Index([1, 2, 3, 4])
>>> idx
Int64Index([1, 2, 3, 4], dtype='int64')

>>> idx.rename('quarter', inplace=True)  # nothing to be returned, just change
>>> idx
Int64Index([1, 2, 3, 4], dtype='int64', name='quarter')

[codecov-io]

Codecov Report

Merging #1134 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1134      +/-   ##
==========================================
+ Coverage   95.22%   95.23%   +<.01%     
==========================================
  Files          35       35              
  Lines        7059     7065       +6     
==========================================
+ Hits         6722     6728       +6     
  Misses        337      337

Impacted Files	Coverage Δ
databricks/koalas/missing/indexes.py	100% <ø> (ø)	⬆️
databricks/koalas/indexes.py	96.73% <100%> (+0.08%)	⬆️

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[HyukjinKwon]

@itholic, can you rebase? seems good.

[itholic]

@HyukjinKwon okay i just rebased it. thanks !