fix: make backend operation asynchronous

[mcollovati]

Description

If the backed connector fails or hangs during serialization enqueue phase, the UI may freeze or behave incorrectly. This change makes all backed operations run asynchronously so that the user is not affected by errors caused by the session storage.

Fixes #114

Type of change

• [x] Bugfix
• [ ] Feature

Checklist

• [x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [x] I have added a description following the guideline.
• [x] The issue is created in the corresponding repository and I have referenced it.
• [x] I have added tests to ensure my change is effective and works as intended.
• [x] New and existing tests are passing locally with my change.
• [ ] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [ ] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.