extindex: speed up --reindex --fast

This required some tweaking of xref3 indices in over.sqlite3,
but the end result is it brings no-op "--reindex --fast --all"
checks down to roughly 20 minutes (from 30-40 minutes) on
lore/all.

This is faster because a bunch of small SQLite queries are still
slower en-mass than a bunch of perlops.  Despite the lack of IPC
overhead, crossing .so boundaries and repeating lookups over
btrees is still slower than doing the same with Perl hash tables.

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