Make the migration lock atomic

[Chee7ah]

Description

This is a follow up to the locking feature from #262 and intends to address the issue pointed out in #262 (comment).

Between checking whether the lock exists and placing the lock, another process could place a lock, which is currently not accounted for. As long as a single process is running the migrations, it is not an issue, but if it is part of multiple processes, we have a race condition.

As for the implementation, the lock document is explicitly inserted with an _id set. This is because upsert operations are still done in two distinct phases on the database side and could end up with duplicate values, unless there is a unique index to prevent this.

If we end up in such a tight race condition between two processes, then because of the unique index already present on the _id field, we'll end up with the DuplicateKey MongoDB error, which is the same case as when the upsert ends up not creating a new document: the lock already exists.

---

Tests are updated to account for the updated implementation of the locking mechanism and also included a new section in the README about the locks, as it didn't really mention how to enable them before.

+ Added a missing await to the TTL index creation to ensure that the index is ready by the time we try to add the lock to it.

Related Issue

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Dependency update
• Other (please describe):

Testing

Checklist

• My code follows the code style of this project (npm run lint passes)
• All tests pass (npm test passes)
• Code coverage remains at 100%
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation (if applicable)
• I have updated the CHANGELOG.md (for user-facing changes)
• My changes generate no new warnings
• I have checked my code for potential security issues

Screenshots (if applicable)

Additional Notes

[tjann]

LGTM