Reduce the thread priority when zone dumping is in progress
After the move of the zone dumping process, we can slightly reduce the scheduling priority of the zone dumping process. This is either done by setting the SCHED_BATCH priority at the beginning and SCHED_OTHER (aka SCHED_NORMAL) back when the zone dumping is done.
(On Windows, this is done by using THREAD_MODE_BACKGROUND_BEGIN and THREAD_MODE_BACKGROUND_END combo.)
Closes #2732 (closed)