add databroker multi lease handlers

[wasaga]

Summary

Adds databroker.NewLeasers convenience method that runs multiple concurrent methods within a single lease.

Related issues

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[coveralls]

Coverage decreased (-0.0002%) to 64.616% when pulling 319fd91 on wasaga/multi-leasers into 5e83691 on main.

[backport-actions-token]

The backport to latest failed:

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

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-latest latest
# Navigate to the new working tree
cd .worktrees/backport-latest
# Create a new branch
git switch --create backport-3255-to-latest
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 443f4a01f54ada22103f7e21736eaf5c6e113176
# Push it to GitHub
git push --set-upstream origin backport-3255-to-latest
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-latest

Then, create a pull request where the base branch is latest and the compare/head branch is backport-3255-to-latest.