The main idea behind it is: the range compaction is very expensive
which can take a few hours to finish. During this long procedure,
a lot of exceptions can occur, e.g.
- Geth is killed manually
- Geth is killed because of machine crash
- etc

In order to minimize the effect of the exceptions, the compaction
is moved out of the pruning. So that even the compaction is not
finished, the pruning is regarded as done.