Fixing ScheduleChangeWorker; updating Jackson

- updated Jackson to version that fixed a bug where nulls were still produced on properties that used a custom (de)serializer, even if this was disabled in the mapper

- Fixed schedule change worker's locking model; now on a lock, waits and tries again without retrying via the executor, and locks are on plan and user. Much more reliable, tested by creating 1000 plans and then deleting them, as fast as possible. Schedules were all deleted correctly.
Eric is saying, 0 is a valid value, if rare, so we shouldn't treat it as "null".