[FLINK-18773][runtime] Enables parallel user-code and plugin class loader.

[AHeise]

What is the purpose of the change

Enables parallel user-code and plugin class loader.

Brief change log

• Mark class loaders as parallel usable.
• Adjust locking in user class loaders (plugin already used the appropriate method).

Verifying this change

No new test (test ideas welcome), covered by all IT and e2e tests.

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, Kubernetes/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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Last check on commit eda9fa6 (Thu Jul 30 13:34:00 UTC 2020)

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[zentol]

Looks fine, had one question for clarity.

[zentol]

what's this change about?

[AHeise]

It's a small performance optimization (should probably go in a separate hotfix commit). The typical class loading pattern indeed has this trailing resolution with an if, but in ChildFirstClassLoader, the resolution already happens at super.loadClassWithoutExceptionHandling(name, resolve). So it should never be necessary to resolve it additionally.

[zentol]

Ok that makes sense, let's move it into another commit and then we're good to merge.