fix: Grab Cornerstone from external in Synchronizer

[swederik]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Fixes the issue that Cornerstone was not being accessed correctly in Synchronizer.js for several functions

• What is the current behavior? (You can also link to an open issue here)

• What is the new behavior (if this is a feature change)?

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

• Other information:

[codecov]

Codecov Report

Merging #1327 (6353cff) into master (18bfdf1) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1327      +/-   ##
==========================================
- Coverage   18.94%   18.94%   -0.01%     
==========================================
  Files         286      286              
  Lines        8672     8673       +1     
  Branches     1486     1486              
==========================================
  Hits         1643     1643              
- Misses       5816     5817       +1     
  Partials     1213     1213              

Impacted Files	Coverage Δ
src/synchronization/Synchronizer.js	0.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 18bfdf1...6353cff. Read the comment docs.

[dannyrb]

🎉 This PR is included in version 5.1.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀