[FLINK-8550][table] Iterate over entryset instead of keys

[hequn8128]

What is the purpose of the change

Iterate over entrysets instead of keys when we want to get both key and value. I went over the code in flink. Luckily, there are not many places we need to optimize.

Brief change log

• Iterate over entrysets instead of keys when we want to get both key and value

Verifying this change

This change is already covered by existing tests, such as
OverWindowHarnessTest for changes in ProcTimeBoundedRangeOver
OverWindowITCase for changes in RowTimeBoundedRangeOver
GroupWindowITCase for changes in JavaUserDefinedAggFunctions
CorrelateITCase for changes in UserDefinedTableFunctions

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)

[fhueske]

Thanks for the PR @hequn8128!
Changes look good, +1 to merge

[zentol]

merging.