Bug 37657: Make koha-preferences CLI tool faster

Bug 37657: Make koha-preferences CLI tool faster

By using Koha::Database->dbh() to use a minimal database handle
which doesn't preload the whole DBIx::Class schema, we're able to
run the same command 2-3 times faster.

This is beneficial when running the tool in a loop which runs the
command serially one by one.

Test plan:

1. time misc/admin/koha-preferences get SearchEngine
2. Note time is about 1 second
3. time misc/admin/koha-preferences dump
4. Note time is about 1 second
5. Create sysprefs.yml

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>