Implement Series.mask()

[itholic]

This PR implement mask of Series
(https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.mask.html#pandas.Series.mask)

>>> s1 = ks.Series([0, 1, 2, 3, 4])
>>> s2 = ks.Series([100, 200, 300, 400, 500])
>>> s1.mask(s1 > 0).sort_index()
0    0.0
1    NaN
2    NaN
3    NaN
4    NaN
Name: 0, dtype: float64

>>> s1.mask(s1 > 1, 10).sort_index()
0     0
1     1
2    10
3    10
4    10
Name: 0, dtype: int64

>>> s1.mask(s1 > 1, s1 + 100).sort_index()
0      0
1      1
2    102
3    103
4    104
Name: 0, dtype: int64

>>> s1.mask(s1 > 1, s2).sort_index()
0      0
1      1
2    300
3    400
4    500
Name: 0, dtype: int64

[codecov-io]

Codecov Report

Merging #1017 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1017      +/-   ##
==========================================
+ Coverage   94.83%   94.83%   +<.01%     
==========================================
  Files          34       34              
  Lines        6640     6641       +1     
==========================================
+ Hits         6297     6298       +1     
  Misses        343      343

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/series.py	96.35% <100%> (ø)	⬆️

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