Introduce df.spark.coalesce()

[xinrong-meng]

Introduce df.spark.coalesce()

[codecov-io]

Codecov Report

Merging #1873 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1873      +/-   ##
==========================================
+ Coverage   94.16%   94.18%   +0.02%     
==========================================
  Files          40       40              
  Lines        9819     9853      +34     
==========================================
+ Hits         9246     9280      +34     
  Misses        573      573              

Impacted Files	Coverage Δ
databricks/koalas/spark/accessors.py	93.95% <100.00%> (+0.20%)	⬆️
databricks/koalas/frame.py	96.72% <0.00%> (-0.04%)	⬇️
databricks/koalas/missing/series.py	100.00% <0.00%> (ø)
databricks/koalas/series.py	96.95% <0.00%> (+0.07%)	⬆️
databricks/koalas/namespace.py	83.01% <0.00%> (+0.21%)	⬆️

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[itholic]

Otherwise, seems good to me