[FLINK-16135][core] Unify the TemporaryClassLoaderContext and AutoContextClassLoader

[StephanEwen]

What is the purpose of the change

These two classes do basically same thing, this PR unifies the use to a consolidated implementation.

The consolidated implementation

• Resides in org.apache.flink.util (not in the plugins sub-package) because it is intended as a general util
• Is called TemporaryClassLoaderContext (better name)
• Uses TemporaryClassLoaderContext.of(classLoader) pattern from the AutoContextClassLoader. This pattern is more in line with the "don't do work in the constructor" coding guide. This also makes it more future-proof, in case we need to do extended handling in the future for specific classloaders (child-first, plugins) or more elaborate handling of thread local variables, etc.

[flinkbot]

Thanks a lot for your contribution to the Apache Flink project. I'm the @flinkbot. I help the community
to review your pull request. We will use this comment to track the progress of the review.

Automated Checks

Last check on commit ea8fa29 (Tue Feb 18 09:43:53 UTC 2020)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

_{Mention the bot in a comment to re-run the automated checks.}

Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

Please see the Pull Request Review Guide for a full explanation of the review process.

The Bot is tracking the review progress through labels. Labels are applied according to the order of the review items. For consensus, approval by a Flink committer of PMC member is required

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot approve description to approve one or more aspects (aspects: description, consensus, architecture and quality)
• @flinkbot approve all to approve all aspects
• @flinkbot approve-until architecture to approve everything until architecture
• @flinkbot attention @username1 [@username2 ..] to require somebody's attention
• @flinkbot disapprove architecture to remove an approval you gave earlier

[flinkbot]

CI report:

• ea8fa29 Travis: FAILURE Azure: FAILURE
• d809478 Travis: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run travis re-run the last Travis build
• @flinkbot run azure re-run the last Azure build

[zentol]

Why @Public? Do we actually expect users to use this?

[StephanEwen]

Not sure. I thought it was a simple enough utility such that it would not hurt to expose it, but we can also conservatively start with @Internal.

[StephanEwen]

Will change the @Public to @Internal for now and merge this, unless there are further objections.