Support for application time periods#347

Apart from versioning, we have two use cases - * AS OF - We need to know entities which were active **as of** given datetime. For an individual entity, I'm able to use transaction.issued_at timestamp to figure out the applicable version. But the query is becoming complicated if I want to fetch a list of rows. * APPLICABLE FROM - When an entity is updated, we need to specify the datetime from which the update will be applicable for the application. Can you suggest best way to handle these requirements? We are ready to make changes to continuum and raise relevant PR if necessary.