[SPARK-3207][MLLIB]Choose splits for continuous features in DecisionTree more adaptively

DecisionTree splits on continuous features by choosing an array of values from a subsample of the data.
Currently, it does not check for identical values in the subsample, so it could end up having multiple copies of the same split. In this PR, we choose splits for a continuous feature in 3 steps:

1. Sort sample values for this feature
2. Get number of occurrence of each distinct value
3. Iterate the value count array computed in step 2 to choose splits.

After find splits, `numSplits` and `numBins` in metadata will be updated.

CC: mengxr manishamde jkbradley, please help me review this, thanks.

Author: Qiping Li <liqiping1991@gmail.com>
Author: chouqin <liqiping1991@gmail.com>
Author: liqi <liqiping1991@gmail.com>
Author: qiping.lqp <qiping.lqp@alibaba-inc.com>

Closes #2780 from chouqin/dt-findsplits and squashes the following commits:

18d0301 [Qiping Li] check explicitly findsplits return distinct splits
8dc28ab [chouqin] remove blank lines
ffc920f [chouqin] adjust code based on comments and add more test cases
9857039 [chouqin] Merge branch 'master' of https://github.com/apache/spark into dt-findsplits
d353596 [qiping.lqp] fix pyspark doc test
9e64699 [Qiping Li] fix random forest unit test
3c72913 [Qiping Li] fix random forest unit test
092efcb [Qiping Li] fix bug
f69f47f [Qiping Li] fix bug
ab303a4 [Qiping Li] fix bug
af6dc97 [Qiping Li] fix bug
2a8267a [Qiping Li] fix bug
c339a61 [Qiping Li] fix bug
369f812 [Qiping Li] fix style
8f46af6 [Qiping Li] add comments and unit test
9e7138e [Qiping Li] Merge branch 'dt-findsplits' of https://github.com/chouqin/spark into dt-findsplits
1b25a35 [Qiping Li] Merge branch 'master' of https://github.com/apache/spark into dt-findsplits
0cd744a [liqi] fix bug
3652823 [Qiping Li] fix bug
af7cb79 [Qiping Li] Choose splits for continuous features in DecisionTree more adaptively
[SPARK-3906][SQL] Adds multiple join support for SQLContext

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

Closes #2767 from liancheng/multi-join and squashes the following commits:

9dc0d18 [Cheng Lian] Adds multiple join support for SQLContext
[SPARK-3800][SQL] Clean aliases from grouping expressions

Author: Michael Armbrust <michael@databricks.com>

Closes #2658 from marmbrus/nestedAggs and squashes the following commits:

862b763 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into nestedAggs
3234521 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into nestedAggs
8b06fdc [Michael Armbrust] possible fix for grouping on nested fields
[SPARK-3966][SQL] Fix nullabilities of Cast related to DateType.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #2820 from ueshin/issues/SPARK-3966 and squashes the following commits:

ca4a745 [Takuya UESHIN] Fix nullabilities of Cast related to DateType.
[SPARK-3945]Properties of hive-site.xml is invalid in running the Thrift JDBC server

Write properties of hive-site.xml to HiveContext when initilize session state in SparkSQLEnv.scala.

The method of SparkSQLEnv.init() in HiveThriftServer2.scala can not write the properties of hive-site.xml to HiveContext. Such as: add configuration property spark.sql.shuffle.partititions in the hive-site.xml.

Author: luogankun <luogankun@gmail.com>

Closes #2800 from luogankun/SPARK-3945 and squashes the following commits:

3679efc [luogankun] [SPARK-3945]Write properties of hive-site.xml to HiveContext when initilize session state In SparkSQLEnv.scala
[SPARK-3969][SQL] Optimizer should have a super class as an interface.

Some developers want to replace `Optimizer` to fit their projects but can't do so because currently `Optimizer` is an `object`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #2825 from ueshin/issues/SPARK-3969 and squashes the following commits:

abbc53c [Takuya UESHIN] Re-rename Optimizer object.
4d2e1bc [Takuya UESHIN] Rename Optimizer object.
9547a23 [Takuya UESHIN] Extract abstract class from Optimizer for developers to be able to replace Optimizer.
[SPARK-3940][SQL] Avoid console printing error messages three times

If  wrong sql,the console print error one times。
eg：
<pre>
spark-sql> show tabless;
show tabless;
14/10/13 21:03:48 INFO ParseDriver: Parsing command: show tabless
............
	at org.apache.spark.sql.hive.thriftserver.SparkSQLCLIDriver.processCmd(SparkSQLCLIDriver.scala:274)
	at org.apache.hadoop.hive.cli.CliDriver.processLine(CliDriver.java:413)
	at org.apache.spark.sql.hive.thriftserver.SparkSQLCLIDriver$.main(SparkSQLCLIDriver.scala:209)
	at org.apache.spark.sql.hive.thriftserver.SparkSQLCLIDriver.main(SparkSQLCLIDriver.scala)
Caused by: org.apache.hadoop.hive.ql.parse.ParseException: line 1:5 cannot recognize input near 'show' 'tabless' '<EOF>' in ddl statement

	at org.apache.hadoop.hive.ql.parse.ParseDriver.parse(ParseDriver.java:193)
	at org.apache.hadoop.hive.ql.parse.ParseDriver.parse(ParseDriver.java:161)
	at org.apache.spark.sql.hive.HiveQl$.getAst(HiveQl.scala:218)
	at org.apache.spark.sql.hive.HiveQl$.createPlan(HiveQl.scala:226)
	... 47 more
Time taken: 4.35 seconds
14/10/13 21:03:51 INFO CliDriver: Time taken: 4.35 seconds
</pre>

Author: wangxiaojing <u9jing@gmail.com>

Closes #2790 from wangxiaojing/spark-3940 and squashes the following commits:

e2e5c14 [wangxiaojing] sql Print the error code three times