[SPARK-3089] Fix meaningless error message in ConnectionManager

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2000 from sarutak/SPARK-3089 and squashes the following commits:

02dfdea [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-3089
e759ce7 [Kousuke Saruta] Improved error message when closing SendingConnection
[SPARK-3128][MLLIB] Use streaming test suite for StreamingLR

Refactored tests for streaming linear regression to use existing  streaming test utilities. Summary of changes:
- Made ``mllib`` depend on tests from ``streaming``
- Rewrote accuracy and convergence tests to use ``setupStreams`` and ``runStreams``
- Added new test for the accuracy of predictions generated by ``predictOnValue``

These tests should run faster, be easier to extend/maintain, and provide a reference for new tests.

mengxr tdas

Author: freeman <the.freeman.lab@gmail.com>

Closes #2037 from freeman-lab/streamingLR-predict-tests and squashes the following commits:

e851ca7 [freeman] Fixed long lines
50eb0bf [freeman] Refactored tests to use streaming test tools
32c43c2 [freeman] Added test for prediction
SPARK-2333 - spark_ec2 script should allow option for existing security group

    - Uses the name tag to identify machines in a cluster.
    - Allows overriding the security group name so it doesn't need to coincide with the cluster name.
    - Outputs the request id's of up to 10 pending spot instance requests.

Author: Vida Ha <vida@databricks.com>

Closes #1899 from vidaha/vida/ec2-reuse-security-group and squashes the following commits:

c80d5c3 [Vida Ha] wrap retries in a try catch block
b2989d5 [Vida Ha] SPARK-2333: spark_ec2 script should allow option for existing security group
Move a bracket in validateSettings of SparkConf

Move a bracket in validateSettings of SparkConf

Author: hzw19900416 <carlmartinmax@gmail.com>

Closes #2012 from hzw19900416/codereading and squashes the following commits:

e717fb6 [hzw19900416] Move a bracket in validateSettings of SparkConf
[SPARK-2790] [PySpark] fix zip with serializers which have different batch sizes.

If two RDDs have different batch size in serializers, then it will try to re-serialize the one with smaller batch size, then call RDD.zip() in Spark.

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

Closes #1894 from davies/zip and squashes the following commits:

c4652ea [Davies Liu] add more test cases
6d05fc8 [Davies Liu] Merge branch 'master' into zip
813b1e4 [Davies Liu] add more tests for failed cases
a4aafda [Davies Liu] fix zip with serializers which have different batch sizes.
[SPARK-3136][MLLIB] Create Java-friendly methods in RandomRDDs

Though we don't use default argument for methods in RandomRDDs, it is still not easy for Java users to use because the output type is either `RDD[Double]` or `RDD[Vector]`. Java users should expect `JavaDoubleRDD` and `JavaRDD[Vector]`, respectively. We should create dedicated methods for Java users, and allow default arguments in Scala methods in RandomRDDs, to make life easier for both Java and Scala users. This PR also contains documentation for random data generation. brkyvz

Author: Xiangrui Meng <meng@databricks.com>

Closes #2041 from mengxr/stat-doc and squashes the following commits:

fc5eedf [Xiangrui Meng] add missing comma
ffde810 [Xiangrui Meng] address comments
aef6d07 [Xiangrui Meng] add doc for random data generation
b99d94b [Xiangrui Meng] add java-friendly methods to RandomRDDs