Fully separate schema: common vs tenant

Consider this a proof of concept and not a formal pull request.

1) maintain separation between "common" and tenant schemas (means that "common" tables are not also in the tenant schema!)
2) taks to process all tenants or a single tenant
3) support for per tenant migrations (potentially scary)
4) changed default so that db:migrate does NOT run  apartment migrations.

There is work left to do to clean things up, for instance apartment:load is missing. Also, tests haven't been updated.