[SPARK-3886] [PySpark] use AutoBatchedSerializer by default

Use AutoBatchedSerializer by default, which will choose the proper batch size based on size of serialized objects, let the size of serialized batch fall in into  [64k - 640k].

In JVM, the serializer will also track the objects in batch to figure out duplicated objects, larger batch may cause OOM in JVM.

Author: Davies Liu <davies.liu@gmail.com>

Closes #2740 from davies/batchsize and squashes the following commits:

52cdb88 [Davies Liu] update docs
185f2b9 [Davies Liu] use AutoBatchedSerializer by default
HOTFIX: Fix build issue with Akka 2.3.4 upgrade.

We had to upgrade our Hive 0.12 version as well to deal with a protobuf
conflict (both hive and akka have been using a shaded protobuf version).
This is testing a correctly patched version of Hive 0.12.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #2756 from pwendell/hotfix and squashes the following commits:

cc979d0 [Patrick Wendell] HOTFIX: Fix build issue with Akka 2.3.4 upgrade.
[SPARK-2924] Required by scala 2.11, only one fun/ctor amongst overriden alternatives, can have default argument(s).

...riden alternatives, can have default argument.

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #2750 from ScrapCodes/SPARK-2924/default-args-removed and squashes the following commits:

d9785c3 [Prashant Sharma] [SPARK-2924] Required by scala 2.11, only one function/ctor amongst overriden alternatives, can have default argument.
[SPARK-3867][PySpark] ./python/run-tests failed when it run with Python 2.6 and unittest2 is not installed

./python/run-tests search a Python 2.6 executable on PATH and use it if available.
When using Python 2.6, it is going to import unittest2 module which is not a standard library in Python 2.6, so it fails with ImportError.

Author: cocoatomo <cocoatomo77@gmail.com>

Closes #2759 from cocoatomo/issues/3867-unittest2-import-error and squashes the following commits:

f068eb5 [cocoatomo] [SPARK-3867] ./python/run-tests failed when it run with Python 2.6 and unittest2 is not installed
[SPARK-3909][PySpark][Doc] A corrupted format in Sphinx documents and building warnings

Sphinx documents contains a corrupted ReST format and have some warnings.

The purpose of this issue is same as https://issues.apache.org/jira/browse/SPARK-3773.

commit: 0e8203f4fb721158fb27897680da476174d24c4b

output
```
$ cd ./python/docs
$ make clean html
rm -rf _build/*
sphinx-build -b html -d _build/doctrees   . _build/html
Making output directory...
Running Sphinx v1.2.3
loading pickled environment... not yet created
building [html]: targets for 4 source files that are out of date
updating environment: 4 added, 0 changed, 0 removed
reading sources... [100%] pyspark.sql
/Users/<user>/MyRepos/Scala/spark/python/pyspark/mllib/feature.py:docstring of pyspark.mllib.feature.Word2VecModel.findSynonyms:4: WARNING: Field list ends without a blank line; unexpected unindent.
/Users/<user>/MyRepos/Scala/spark/python/pyspark/mllib/feature.py:docstring of pyspark.mllib.feature.Word2VecModel.transform:3: WARNING: Field list ends without a blank line; unexpected unindent.
/Users/<user>/MyRepos/Scala/spark/python/pyspark/sql.py:docstring of pyspark.sql:4: WARNING: Bullet list ends without a blank line; unexpected unindent.
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] pyspark.sql
writing additional files... (12 module code pages) _modules/index search
copying static files... WARNING: html_static_path entry u'/Users/<user>/MyRepos/Scala/spark/python/docs/_static' does not exist
done
copying extra files... done
dumping search index... done
dumping object inventory... done
build succeeded, 4 warnings.

Build finished. The HTML pages are in _build/html.
```

Author: cocoatomo <cocoatomo77@gmail.com>

Closes #2766 from cocoatomo/issues/3909-sphinx-build-warnings and squashes the following commits:

2c7faa8 [cocoatomo] [SPARK-3909][PySpark][Doc] A corrupted format in Sphinx documents and building warnings
Merge branch 'master' of git://git.apache.org/spark into SPARK-3900