add setSync function, write user data sync.

[zhenshuaiwws]

When I want to save data before the app exits, but the set method does not complete before the end of the process, resulting in the data not saved successfully, so I need to save the data synchronously.

[jviotti]

Hey there, thanks a lot for the contribution! Some comments:

• Can we add tests for this new function?
• Since this is a synchronous function, it shouldn't take a callback, but simply return the value