scanner import db is slow, improve its performances

As per title, currently importing a list of ~30 M SWHIDs to an sqlite database using swh scanner db import takes about 30 minutes (tested on granet). It can very likely be much faster. We should try the following classic approaches to make bulk import into SQLite faster:

• use pragma synchronous = off (yes, it is unsafe, but the conversion to sqlite is all of nothing anyway, so it's not a factor here)
• use pragma journal_mode = off (ditto)
• wrap all imports in a single transaction
• verify we are using prepared statements for inserts

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Migrated from T2812 (view on Phabricator)