When we rewrote the zone dumping to use the separate threadpool, the
dumping would acquire the read lock for the whole time the zone dumping
process is dumping the zone.

When combined with incoming IXFR that tries to acquire the write lock on
the same rwlock, we would end up blocking all the other readers.

In this commit, we pause the dbiterator every time we get next record
and before start dumping it to the disk.