Adding support to fetch changes from Lucene store while migrating from/to r…

[astute-decipher]

…emote store to/from Docrep.

Description

In today's scenario, follower index fetch changes from leader index through translog if the leader cluster is DocRep based and through lucene store if the leader cluster is RemoteStore based. In case of mixed mode, when cluster is migrating from DocRep to RemoteStore or vice versa , we want to fetch the changes from lucene store only.

Issues Resolved

Resolves #1360

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[gbbafna]

Looks good

[ankitkala]

Overall changes looks good to me!
Have few minor comments.

[opensearch-trigger-bot]

The backport to 2.x 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.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-1369-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ff42012067e0b14e0e15b79c9be542cbdff08031
# Push it to GitHub
git push --set-upstream origin backport/backport-1369-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-1369-to-2.x.