[FLINK-14962] fix findbugs WMI_WRONG_MAP_ITERATOR warning

[caidezhi]

What is the purpose of the change

This PR fixes WMI_WRONG_MAP_ITERATOR warnings reported by FindBugs.

Brief change log

use an iterator on the entrySet of the map, to avoid the Map.get(key) lookup.
related class :
org.apache.flink.formats.parquet.ParquetMapInputFormat
org.apache.flink.graph.library.CommunityDetection
org.apache.flink.streaming.connectors.fs.bucketing.BucketingSink (already marked as "Deprecated")

Verifying this change

This change is already covered by existing tests.

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)

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Automated Checks

Last check on commit 05b6cbc (Wed Dec 04 15:14:27 UTC 2019)

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CI report:

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[KurtYoung]

Thanks for improving this @caidezhi

[KurtYoung]

Please keep the comment.

[caidezhi]

Kurt, thanks for reviewing. I add back the original comments as you suggested.