xcpdb|compact: support --jobs/-j flag like gmake(1)
We don't have to be tied to the number of partitions in case
we made a bad choice at initialization. This doesn't affect
reindexing, but the copying phase is already intensive.
And optimize away the extra process when we only have a single
job which won't parallelize.
The wording for the (v2) reindexing phase could be improved,
later. I also plan to allow repartitioning of existing
Xapian DBs.