WIP: Dbsync performance improvements

[Siedlerchr]

I could not push to the original one Follow up to #8494

Introduced a dummy FXThread method for the CurrentTaskExecutor to fix the tests and added some synchronized to prevent errors with transactions. Need to check if we need this for other udpates as well

• [ ] Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• [x] Tests created for changes (if applicable)
• [x] Manually tested changed features in running JabRef (always required)
• [ ] Screenshots added in PR description (for UI changes)
• [ ] Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

This need some more thinking how to solve blocking threads when the EventBus fires. Therefore I close this for the moment and reopen when I have more insights