Implement rev query parameter for HEAD, GET, PUT and DELETE requests.

Instead of using `entity_load()` it's better if we add the `rev` key to `\Drupal\multiversion\Entity\UuidIndex::buildValue`. Because then we can do this instead, which is MUCH faster:

    $record = \Drupal::service('entity.uuid_index')->get($entity->uuid());
    $last_rev = $record['rev'];

So please update `\Drupal\multiversion\Entity\UuidIndex::buildValue` and add `rev` (from `$entity->_revs_info->rev`) and `revision_id` (from `$entity->getRevisionId()`).

Then we also need to make sure that the UUID index is being kept up to date *all the time* (insert, update and delete), because right now it's only update on `hook_entity_insert` and `hook_entity_delete` in `multiversion.module`.