Additional thread synchronization in the AggregateProcessor to prevent orphaned aggregate groups

[dlvenable]

Description

We are seeing cases where aggregations are not acknowledging completion. From investigation, I can see that there are places where we can have orphaned aggregate groups. Basically, some code paths can create two aggregate groups and only one remains in the map.

This PR adds tight synchronization on each AggregateProcessor to prevent these possible duplicates or orphans.

We may be able to improve this later. But, for now, this should fix the current issue.

Issues Resolved

N/A

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • New functionality has javadoc added
• Commits are signed with a real name per the DCO

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[opensearch-trigger-bot]

The backport to 2.13 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.13 2.13
# Navigate to the new working tree
cd .worktrees/backport-2.13
# Create a new branch
git switch --create backport/backport-6439-to-2.13
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 72ad01221742716c4c1de8ea0b095cf0cf3dd682
# Push it to GitHub
git push --set-upstream origin backport/backport-6439-to-2.13
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.13

Then, create a pull request where the base branch is 2.13 and the compare/head branch is backport/backport-6439-to-2.13.