[FLINK-25987][state/changelog] Replace lastSqn with nextSqn()

[rkhachatryan]

What is the purpose of the change

The current semantics of ChangelogWriter.lastAppendedSqn doesn't allow
to distinguish between initial empty and non-empty states.
This results in wrong SQN passed to truncate and persist methods.

This change solves this by replacing lastAppendedSqn with nextSqn.
It essentially moves the responsibility to call sqn.next() on materialization
from backend to writer which has the knowledge of whether there were any changes or not.

Verifying this change

ProcessingTimeWindowCheckpointingITCase

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, Kubernetes/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? no

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Automated Checks

Last check on commit 0915277 (Fri Feb 11 16:01:05 UTC 2022)

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[Myasuka]

Thanks for creating this PR!
LGTM on the whole.