getSyncCursor
Returns the maximum stored server write timestamp for this user, including deleted rows.
The cursor uses server nanoseconds rather than device edit times. It is not a complete-batch checkpoint: interruption while applying records sharing a timestamp can skip unapplied peers on the next strict-greater-than pull.
Return
Maximum server timestamp in epoch nanoseconds, or null when no rows exist.
Parameters
userId
Owner whose stored timestamps are examined.