lei_xsearch: use per-external queries when not sorting

We only need the combined mset query when we care about sort
order.  When writing to --output destinations intended for MUA
consumption, sort order is irrelevant as MUAs are expected to
offer their own sorting, so run queries to each external in
parallel.

This prepares us for docid-sort-based saved search support.

It will also become faster than the combined mset query for
users with many externals due to current Xapian exhibiting poor
performance with many shards (the same reason -extindex exists)

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