Each app insert required asking the database for the ID of each
category an app is in. Given the categories don't change (ever)
but are only appended to, we can cache the results in a static
Java variable for increased performance.
This reduced the "repo persister" logic for me from 50 seconds
for main F-Droid and 100 seconds for Archive, down to 15 seconds
and 30 seconds respectively.
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