upgrade tap_postgres to elegantly handle backfills for gitlab_com (and other internal data sources like it)

The gitlab_com data needs to get pulled in a few different ways. In some cases, tables can be backfilled normally using their updated_at column, in other cases though they might be append only and in others still might be slowly changing facts that should be fully loaded on every run. need to figure out what table needs what and then update tap_postgres accordingly to be able to handle this array of loading possibilities:

table	loading_method
approvals	updated_at
approver_groups	updated_at
audit_events	updated_at