Pagination in Storage/Central

The storage and central API should accept "page" and "per_page" values.

Additionally, we need to amend the listing methods to return some additional metadata like "total_items" etc

Mock a central method for finding domains:

    def find_domains(self, context, criterion=None, page=None, per_page=None):

        # ....
        domains = [{
            'id': '12345',
            'name': 'example.com.',
        }, {
            'id': '54321',
            'name': 'example.org.',
        }]

        metadata = {
            'total_items': 50,
            'has_next': True
        }

        return domains, metadata

Mock usage:

    domains, metadata = central_api.find_domains(context, page=1, per_page=30)