func NewActivityIterator(results []*vocab.ActivityType, totalItems int) *ActivityIterator {
	return &ActivityIterator{
		iterator: newIterator(totalItems),
		results:  results,
	}
}
NewActivityIterator creates a new ActivityIterator.
package memstore
import (
	"net/url"
	"github.com/trustbloc/orb/pkg/activitypub/store/spi"
	"github.com/trustbloc/orb/pkg/activitypub/vocab"
)
type iterator struct {
	current    int
	totalItems int
}
func newIterator(totalItems int) *iterator {
	return &iterator{
		totalItems: totalItems,
		current:    -1,
	}
}
func (it *iterator) TotalItems() (int, error) {
	return it.totalItems, nil
}
func (it *iterator) Close() error {
	return nil
}
type ActivityIterator struct {
	*iterator
	results []*vocab.ActivityType
}