[FLINK-10693] [Scala API] Fix incorrect duplication in EitherSerializer

[StephanEwen]

What is the purpose of the change / Brief change log

Fixes the duplication of the Scala EitherSerialzer that never deep copies, even though the nested serializers might require it.

Verifying this change

• Adds a self-contained unit test

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: yes
• 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

[StephanEwen]

CI test failure is unrelated:
FlinkKafkaProducerITCase.testFlinkKafkaProducer10FailTransactionCoordinatorBeforeNotify -Timeout

[tillrohrmann]

Thanks for the fix @StephanEwen. LGTM. I think this fix should also be backported to release-1.6 and release-1.5.

[tillrohrmann]

I'll backport this PR to release-1.5 and release-1.6. If Travis passes, then I'll merge it.