[FLINK-14118][benchmarks]Add network throughput benchmark for data skew scenario.

[wsry]

What is the purpose of the change

The purpose of this pr is to add a network benchmark for data skew scenario in which most of the data will go to a few channels. The added benchmark simulates the most extreme scenario in which nearly all data goes to only one channel.

Brief change log

• Add DataSkewStreamNetworkThroughputBenchmark to simulate data skew scenario.
• Add DataSkewStreamNetworkThroughputBenchmarkTest to verify the added benchmark.

Verifying this change

A new DataSkewStreamNetworkThroughputBenchmarkTest was added to verify the change.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Automated Checks

Last check on commit b344a21 (Wed Oct 16 08:37:55 UTC 2019)

Warnings:

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• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• b344a21 : CANCELED Build

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[pnowojski]

LGTM. I'll merge it soon.

One note for the future, please also follow our convention of naming the commits (in this case missing [FLINK-14118][benchmarks] prefix)