[FLINK-13800][State Backends] Add maven module for spill-able heap backend

[carp84]

What is the purpose of the change

Adding a new sub-module under flink-state-backends for the spill-able heap backend as proposed by FLIP-50.

Brief change log

Adding the directory and pom file for the new sub module.

Verifying this change

This change is a maven related change without any newly introduced test, won't affect any existing test either.

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

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

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

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Automated Checks

Last check on commit 7c52054 (Fri Aug 23 10:22:51 UTC 2019)

Warnings:

• 2 pom.xml files were touched: Check for build and licensing issues.
• No documentation files were touched! Remember to keep the Flink docs up to date!

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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:

• 7c52054 : CANCELED Build

[tillrohrmann]

Thanks for this PR @carp84. The changes look good to me. I will resolve my own comment while merging the PR by removing the dependencies which we don't need at the moment.

[tillrohrmann]

I guess we could remove the dependencies for the moment because without code, it is hard to tell which dependency is actually needed. Once we add the code, we can re-add the required dependencies.