[SPARK-4493][SQL] Tests for IsNull / IsNotNull in the ParquetFilterSuite

This is a follow-up of #3367 and #3644.

At the time #3644 was written, #3367 hadn't been merged yet, thus `IsNull` and `IsNotNull` filters are not covered in the first version of `ParquetFilterSuite`. This PR adds corresponding test cases.

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Author: Cheng Lian <lian@databricks.com>

Closes #3748 from liancheng/test-null-filters and squashes the following commits:

1ab943f [Cheng Lian] IsNull and IsNotNull Parquet filter test case for boolean type
bcd616b [Cheng Lian] Adds Parquet filter pushedown tests for IsNull and IsNotNull
[SPARK-4916][SQL][DOCS]Update SQL programming guide about cache section

`SchemeRDD.cache()` now uses in-memory columnar storage.

Author: luogankun <luogankun@gmail.com>

Closes #3759 from luogankun/SPARK-4916 and squashes the following commits:

7b39864 [luogankun] [SPARK-4916]Update SQL programming guide
6018122 [luogankun] Merge branch 'master' of https://github.com/apache/spark into SPARK-4916
0b93785 [luogankun] [SPARK-4916]Update SQL programming guide
99b2336 [luogankun] [SPARK-4916]Update SQL programming guide
[SPARK-4930][SQL][DOCS]Update SQL programming guide, CACHE TABLE is eager

`CACHE TABLE tbl` is now __eager__ by default not __lazy__

Author: luogankun <luogankun@gmail.com>

Closes #3773 from luogankun/SPARK-4930 and squashes the following commits:

cc17b7d [luogankun] [SPARK-4930][SQL][DOCS]Update SQL programming guide, add CACHE [LAZY] TABLE [AS SELECT] ...
bffe0e8 [luogankun] [SPARK-4930][SQL][DOCS]Update SQL programming guide, CACHE TABLE tbl is eager
[SPARK-4928][SQL] Fix: Operator '>,<,>=,<=' with decimal between different precision report error

case operator  with decimal between different precision, we need change them to unlimited

Author: guowei2 <guowei2@asiainfo.com>

Closes #3767 from guowei2/SPARK-4928 and squashes the following commits:

c6a6e3e [guowei2] fix code style
3214e0a [guowei2] add test case
b4985a2 [guowei2] fix code style
27adf42 [guowei2] Fix: Operation '>,<,>=,<=' with Decimal report error