searchidx: split shards at 100000 docs by default

searchidx: split shards at 100000 docs by default

Testing on a busy btrfs system with indexlevel=medium reveals
another ~15% speedup compared to the previous 450000 value since
shards are smaller and less prone to slowdown.  The smaller
splits should also work better with indexlevel=full (the
default) since full indexing with positions takes up the bulk of
the space.