extindex: support --jobs/-j properly on creation for shard count
This wasn't wired up properly, but Xapian appears to suffer from
I/O amplification problems as DB shards get larger:
https://lists.xapian.org/pipermail/xapian-discuss/2019-February/009727.html
<23640.32170.703368.841021@y.dockes.com>
Of course, we shouldn't have too many shards, either; because
performance problems with too many shards was the entire reason
extindex was created:
https://lists.xapian.org/pipermail/xapian-discuss/2020-August/009823.html
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20200826064728.GA32239@dcvr>