Implement the /db/_revs_diff endpoint

We need to create a normalizer (something like \Drupal\relaxed\Normalizer\RevsDiffNormalizer) and a new resource endpoint (something like \Drupal\relaxed\Plugin\rest\RevsDiffResource).

The resource should follow the following specification: http://docs.couchdb.org/en/latest/api/database/misc.html#db-revs-diff

To clarify the CouchDB documentation; In the example below the dictionary key (190f721ca3411be7aa9477db5f948bbb) is the document ID for the given revision IDs. This makes it so that you can post multiple document/revision combinations to the endpoint.

POST /db/_revs_diff HTTP/1.1
Accept: application/json
Content-Length: 113
Content-Type: application/json
Host: localhost:5984

{
  "190f721ca3411be7aa9477db5f948bbb": [
    "3-bb72a7682290f94a985f7afac8b27137",
    "4-10265e5a26d807a3cfa459cf1a82ef2e",
    "5-067a00dff5e02add41819138abb3284d"
  ]
}

We should use the \Drupal\multiversion\Entity\RevisionIndex to look up all the revisions. See this example of how it's used: https://github.com/dickolsson/drupal-relaxed/blob/8.x-1.x/src/ParamConve...

We can skip the possible_ancestors key for now and deal with that in #2344005: Implement the possible_ancestors key

For the normalizer we should create a new service class, something like this (totally untested):

class RevisionDiff implements RevisionDiffInterface {

  public function __construct(RevisionIndex $rev_index) {
    $this->revisionIndex = $rev_index;
  }

  public function entities(array $entity_keys) {
    $this->entityKeys = $entity_keys;
    return $this;
  }

  public function getMissing() {
    $keys = array();
    foreach ($this->EntityKeys as $entity_uuid => $revision_ids) {
      foreach ($revision_ids as $revison_id) {
        $keys[] = $entity_uuid . ':' . $revision_id;
      }
    }
    $existing_revision_ids = $this->revisionIndex->getMultiple($keys);
    // Do diff.
    return $missing_revision_ids;
  }
}

With this one could then do:

$entity_keys = array(
  '123-some-uuid-456' => array(
    '1-123-some-revision-id-567',
    '2-890-some-revision-id-123',
    // ...
  ),
);
$missing = $revDiff->entities($entity_keys)->getMissing();