[SPARK-2908] [SQL] JsonRDD.nullTypeToStringType does not convert all NullType to StringType

JIRA: https://issues.apache.org/jira/browse/SPARK-2908

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1840 from yhuai/SPARK-2908 and squashes the following commits:

86e833e [Yin Huai] Update test.
cb11759 [Yin Huai] nullTypeToStringType should check columns with the type of array of structs.
[SPARK-2877] [SQL] MetastoreRelation should use SparkClassLoader when creating the tableDesc

JIRA: https://issues.apache.org/jira/browse/SPARK-2877

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1806 from yhuai/SPARK-2877 and squashes the following commits:

4142bcb [Yin Huai] Use Spark's classloader.
[SPARK-2919] [SQL] Basic support for analyze command in HiveQl

The command we will support is
```
ANALYZE TABLE tablename COMPUTE STATISTICS noscan
```
Other cases shown in https://cwiki.apache.org/confluence/display/Hive/StatsDev#StatsDev-ExistingTables will still be treated as Hive native commands.

JIRA: https://issues.apache.org/jira/browse/SPARK-2919

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1848 from yhuai/sqlAnalyze and squashes the following commits:

0b79d36 [Yin Huai] Typo and format.
c59d94b [Yin Huai] Support "ANALYZE TABLE tableName COMPUTE STATISTICS noscan".