Add PyArrow 0.15 support back, and test PyArrow 0.16 in CI

[HyukjinKwon]

This PR adds PyArrow 0.15 support back with a better documentation and error message.
Error message will look as below:

Warning when pyarrow>=0.15 and pyspark<3.0 but ARROW_PRE_0_15_IPC_FORMAT not set.

WARNING:root:'ARROW_PRE_0_15_IPC_FORMAT' environment variable was not set. It is required 
to set this environment variable to '1' if you use pyarrow>=0.15 and pyspark<3.0. Koalas will 
set it for you but it does not work if there is a Spark context already launched.

Exceptions when ARROW_PRE_0_15_IPC_FORMAT is set in valid cases:

RuntimeError: Please explicitly unset 'ARROW_PRE_0_15_IPC_FORMAT' environment variable 
in both driver and executor sides. Check your spark.executorEnv.*, spark.yarn.appMasterEnv.*, 
spark.mesos.driverEnv.* and spark.kubernetes.driverEnv.* configurations. It is required to set 
this environment variable only when you use pyarrow>=0.15 and pyspark<3.0.

RuntimeError: Please explicitly unset 'ARROW_PRE_0_15_IPC_FORMAT' environment variable in 
both driver and executor sides. It is required to set this environment variable only when you use 
pyarrow>=0.15 and pyspark<3.0.

Resolves #1109

[codecov-io]

Codecov Report

Merging #1110 into master will increase coverage by 0.03%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #1110      +/-   ##
==========================================
+ Coverage   95.01%   95.05%   +0.03%     
==========================================
  Files          34       34              
  Lines        7944     7962      +18     
==========================================
+ Hits         7548     7568      +20     
+ Misses        396      394       -2     

Impacted Files	Coverage Δ
databricks/koalas/__init__.py	92.30% <71.42%> (-1.31%)	⬇️
databricks/koalas/utils.py	96.92% <87.50%> (+1.71%)	⬆️
databricks/koalas/base.py	97.57% <0.00%> (+0.06%)	⬆️

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[ueshin]

Otherwise, LGTM.

[HyukjinKwon]

Thanks for detailed review @ueshin!

[ueshin]

LGTM.

[ueshin]

Thanks! merging.