feat: Synchronize only refreshes to preserve multi-threading

[bflorian]

Synchronize only refreshing of tokens that that the initial requests when an accessToken expires. This way callbacks can be made in parallel as long as the tokens are valid.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document
• [x] My code follows the code style of this project (hint: install an xo editor plugin)
• [ ] Any required documentation has been added
• [ ] I have added tests to cover my changes