Function rehydration_finished 

pub fn rehydration_finished<'scope, T: Timestamp>(
    scope: Scope<'scope, T>,
    source_config: &RawSourceCreationConfig,
    token: impl Any + 'static,
    resume_upper: Antichain<T>,
    input: StreamVec<'scope, T, Infallible>,
)

This leaf operator drops token after the input reaches the resume_upper. This is useful to take coordinated actions across all workers, after the upsert operator has rehydrated.