Increase vacuum threshold to 1%, and analyze threshold to 5%

From #129 (comment 93840341):

I think it's definitely fine for the migration, but then I'd raise vacuum threshold to 1% and analyze threshold to 5% or some other values, based on analysis of autovacuum activity. Such analysis is worth to do after the GCP migration and after upgrading Postgres to 9.6.10 due to this fix (from the 9.6.9's changelog https://www.postgresql.org/docs/current/static/catalog-pg-class.html):

Change ANALYZE's algorithm for updating pg_class.reltuples (David Gould)

Previously, pages not actually scanned by ANALYZE were assumed to retain their old tuple density. In a large table where ANALYZE samples only a small fraction of the pages, this meant that the overall tuple density estimate could not change very much, so that reltuples would change nearly proportionally to changes in the table's physical size (relpages) regardless of what was actually happening in the table. This has been observed to result in reltuples becoming so much larger than reality as to effectively shut off autovacuuming. To fix, assume that ANALYZE's sample is a statistically unbiased sample of the table (as it should be), and just extrapolate the density observed within those pages to the whole table.