Enabled Async Shard Batch Fetch by default

[gargmanik13]

Description

Enabled Async Shard Batch Fetch by default

• Updated default values for primary_allocator_timeout and replica_allocator_timeout to 20s.

Related Issues

Resolves #17713
Known issues - #18098

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

✅ Gradle check result for 845605a: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.55%. Comparing base (f55af4e) to head (845605a).
Report is 15 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #18139      +/-   ##
============================================
- Coverage     72.56%   72.55%   -0.01%     
- Complexity    67227    67251      +24     
============================================
  Files          5476     5476              
  Lines        310445   310446       +1     
  Branches      45124    45124              
============================================
- Hits         225261   225242      -19     
- Misses        66786    66851      +65     
+ Partials      18398    18353      -45     

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[opensearch-trigger-bot]

The backport to 3.0 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-3.0 3.0
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-3.0
# Create a new branch
git switch --create backport/backport-18139-to-3.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 0cbd84854b7eda5f7d8c1a030c265528da275b6f
# Push it to GitHub
git push --set-upstream origin backport/backport-18139-to-3.0
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-3.0

Then, create a pull request where the base branch is 3.0 and the compare/head branch is backport/backport-18139-to-3.0.