[FLINK-9784] Fix inconsistent use of 'static' in AsyncIOExample.java

[nekrassov]

What is the purpose of the change

This patch fixes inconsistent use of static in AsyncIOExample.java that can lead to incorrect code execution.

Brief change log

Changed executorService and random from static to instance-specific, removed no longer needed counter, and removed synchronization on class that's no longer needed either.

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? (not applicable)

[yanghua]

+1, from my side

[tillrohrmann]

LGTM. Thanks for this fix @nekrassov and the review @yanghua. Merging this PR.