HDDS-13874. Get rid of SnapshotCompactionDag

[echonesis]

What changes were proposed in this pull request?

This PR refactors the snapshot compaction tracking mechanism by replacing SnapshotCompactionDag with a new flush-based tracking system using FlushLinkedList and related components.

Major changes:

• Removed: CompactionDag and CompactionNode classes that tracked compaction operations in a DAG structure
• Added: Flush tracking system:
  • FlushLinkedList: Tracks flush operations in a linked list structure
  • FlushNode: Represents a single flush operation with snapshot generation metadata
  • FlushLogEntry: DAO for writing flush log entries to the flush log table
  • FlushFileInfo: Metadata for flushed SST files

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-13874

How was this patch tested?

GitHub Actions CI: https://github.com/echonesis/ozone/actions/runs/21389340168

[adoroszlai]

@aswinshakil @sadanand48 @smengcl please take a look

[peterxcli]

I think we need a design document for this or at least more description and diagram

[aswinshakil]

This is a major change. As @peterxcli mentioned, We need some design doc to understand and review this change. @echonesis Can you please add them?

[echonesis]

Thanks @peterxcli and @aswinshakil for the suggestion!
I agree that a design doc is necessary, and I'm more than willing to draft one.
Could you point me to any templates you recommend?
That would definitely help me get started on the right track.

[aswinshakil]

You can look at these design PRs for better understanding. These would give you an idea for adding design doc for new feature/changes to existing ones.