Implement Series.asof()

[itholic]

This PR proposes Series.asof()
(https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.asof.html#pandas.Series.asof)

>>> s = ks.Series([1, 2, np.nan, 4], index=[10, 20, 30, 40])
>>> s
10    1.0
20    2.0
30    NaN
40    4.0
Name: 0, dtype: float64

A scalar where.

>>> s.asof(20)
2.0

For a sequence where, a Series is returned. The first value is
NaN, because the first element of where is before the first
index value.

>>> s.asof([5, 20]).sort_index()
5     NaN
20    2.0
Name: 0, dtype: float64

Missing values are not considered. The following is 2.0, not
NaN, even though NaN is at the index location for 30.

>>> s.asof(30)
2.0

[codecov-io]

Codecov Report

Merging #1366 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1366      +/-   ##
==========================================
+ Coverage   95.17%   95.19%   +0.01%     
==========================================
  Files          34       34              
  Lines        7878     7902      +24     
==========================================
+ Hits         7498     7522      +24     
  Misses        380      380              

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100.00% <ø> (ø)
databricks/koalas/series.py	96.96% <100.00%> (+0.09%)	⬆️

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[HyukjinKwon]

Seems fine. Can you rebase @itholic?