[SPARK-5404] [SQL] Update the default statistic number

[chenghao-intel]

By default, the statistic for logical plan with multiple children is quite aggressive, and those statistic are quite critical for the join optimization, hence we need to estimate the statistics as accurate as possible.

For Union, which has 2 children, and overwrite the default implementation by adding its children byteInSize instead of multiplying.
For Expand, which only has a single child, but it will grows the size, and we need to multiply its inflating factor.

[SparkQA]

Test build #28295 has started for PR 4914 at commit d466bbc.

• This patch merges cleanly.

[SparkQA]

Test build #28295 has finished for PR 4914 at commit d466bbc.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/28295/
Test PASSed.

[marmbrus]

Please explain you changes.

[chenghao-intel]

Thank you @marmbrus I've updated the description.

[marmbrus]

Thanks! Merging to master.