When preparing a temp database to write to, don't copy all apps/apks.
Instead, only copy those _not_ belonging to the repo we are updating.
In an ideal world, we'd not even need to copy them, but we need
their IDs to be in the temp database so that we don't accidentally
use the same auto-generated ID as the main database.
This also means that we can drop the check for "does this app exist,
and hence should we UPDATE it instead of INSERTing it?" and always
just insert it.
Then, when copying the temp table back to disk, first delete all
apps/apks _belonging to the repo being updated_. Then, copy back the
apks/apps we found in the repo. This again improves performance because
we no longer need to bopy back and forth data which we know
wont change (as evidenced by the fact it belongs to a differen trepo).
I don't think this was possible earlier before we did the work to
support repo priorities properly. That is because we had a single app
which was serviced by several repositories. Now, we have multiple
entries in the `fdroid_app` table, for each repo which supports
that app.
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