[FLINK-23314][cep] State left behind for short lived keys

[dawidwys]

What is the purpose of the change

Properly clean up state in CepOperator

Brief change log

Couple of improvements for state cleaning in the CepOperator.

• register timer in order to advance time, if we keep count of records
  for certain timestamps
• clear the namespace if there are no more timestamps we keep count of
  events for
• clear up NfaState if there is only starting partial match

Verifying this change

This is a Work in Progress, needs a test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Last check on commit 5b0d1da (Thu Jul 08 12:15:03 UTC 2021)

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[mayuehappy]

Hello, is there any possibility of not registering the timer here?
I may have found a performance problem caused by registering the timer, which may have something to do with the code in this place. Can you take a look at this issue if possible? https://issues.apache.org/jira/browse/FLINK-23890