Fix state file truncation detection

[jefchien]

Description of the issue

The recent state manager refactor introduced an issue where the state file no longer resets when a tailed file is truncated. The problem was that on restart, the state file was read and used to determine where to seek to in the file, but that same offset was not used as a starting point for the state file management. So, it was unable to determine that the new offset was smaller than the restored one.

Steps to reproduce

1. Write logs

date >> /tmp/log/test
date >> /tmp/log/test
date >> /tmp/log/test

2. Truncate the logs

truncate /tmp/log/test --size 0

3. Restart the agent
4. Write again (the state file will not get updated)

date >> /tmp/log/test

6. Restart the agent (it will send the same log again as the state will default to 0)

Description of changes

Changes the state file restoration so the restored offset gets tracked.

License

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Tests

Added unit tests. Before the fix, the newly added TestLogsFileTruncateRestart would fail because it would not have used the restored offset from the state file to detect truncation.

Requirements

Before commiting your code, please do the following steps.

1. Run make fmt and make fmt-sh
2. Run make lint

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Integration Tests

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