[MLLIB] use Iterator.fill instead of Array.fill

Iterator.fill uses less memory

Author: Xiangrui Meng <meng@databricks.com>

Closes #1930 from mengxr/rand-gen-iter and squashes the following commits:

24178ca [Xiangrui Meng] use Iterator.fill instead of Array.fill
[SPARK-3004][SQL] Added null checking when retrieving row set

JIRA issue: [SPARK-3004](https://issues.apache.org/jira/browse/SPARK-3004)

HiveThriftServer2 throws exception when the result set contains `NULL`. Should check `isNullAt` in `SparkSQLOperationManager.getNextRowSet`.

Note that simply using `row.addColumnValue(null)` doesn't work, since Hive set the column type of a null `ColumnValue` to String by default.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1920 from liancheng/spark-3004 and squashes the following commits:

1b1db1c [Cheng Lian] Adding NULL column values in the Hive way
2217722 [Cheng Lian] Fixed SPARK-3004: added null checking when retrieving row set
[SPARK-2817] [SQL] add "show create table" support

In spark sql component, the "show create table" syntax had been disabled.
We thought it is a useful funciton to describe a hive table.

Author: tianyi <tianyi@asiainfo-linkage.com>
Author: tianyi <tianyi@asiainfo.com>
Author: tianyi <tianyi.asiainfo@gmail.com>

Closes #1760 from tianyi/spark-2817 and squashes the following commits:

7d28b15 [tianyi] [SPARK-2817] fix too short prefix problem
cbffe8b [tianyi] [SPARK-2817] fix the case problem
565ec14 [tianyi] [SPARK-2817] fix the case problem
60d48a9 [tianyi] [SPARK-2817] use system temporary folder instead of temporary files in the source tree, and also clean some empty line
dbe1031 [tianyi] [SPARK-2817] move some code out of function rewritePaths, as it may be called multiple times
9b2ba11 [tianyi] [SPARK-2817] fix the line length problem
9f97586 [tianyi] [SPARK-2817] remove test.tmp.dir from pom.xml
bfc2999 [tianyi] [SPARK-2817] add "File.separator" support, create a "testTmpDir" outside the rewritePaths
bde800a [tianyi] [SPARK-2817] add "${system:test.tmp.dir}" support add "last_modified_by" to nonDeterministicLineIndicators in HiveComparisonTest
bb82726 [tianyi] [SPARK-2817] remove test which requires a system from the whitelist.
bbf6b42 [tianyi] [SPARK-2817] add a systemProperties named "test.tmp.dir" to pass the test which contains "${system:test.tmp.dir}"
a337bd6 [tianyi] [SPARK-2817] add "show create table" support
a03db77 [tianyi] [SPARK-2817] add "show create table" support