[FLINK-4387][QS][tests] don't wait and process requests at Netty servers after shutdown request

[NicoK]

What is the purpose of the change

There is a race condition on an assertion in Netty's event loop that may cause
tests to fail when finished early.

This was fixed in 4.0.33.Final via netty/netty#4357 but we currently still rely on Netty 4.0.27.Final, hence the bug fix for the tests where this bug can be immanent.

Brief change log

• in QueryableState tests, do not wait for a "quiet period" for further events to arrive and process, this is not needed here and circumvents the bug

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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? JavaDocs

[kl0u]

LGTM +1 to merge.