incrementally by successive inserts rather than by sorting the data.
We were only using the slow path during bootstrap, apparently because
when first written it failed during bootstrap --- but it works fine now
AFAICT.  Removing it saves a hundred or so lines of code and produces
noticeably (~10%) smaller initial states of the system catalog indexes.
While that won't make much difference for heavily-modified catalogs,
for the more static ones there may be a useful long-term performance
improvement.