Fix getItems() performance issue with RedisCluster (php-redis)

On any kind of multi loads, including tags loading, current code leads to
an explosion of Redis lookups slowing down performance.

This backports the code for mget() usage from 4.x in order to fix it.
It's done with one small improvment which would also be relevant for 4.x,
only using pipeline on cluster on predis as mget is more efficient.

src/Symfony/Component/Cache/Traits/RedisTrait.php

@@ -169,18 +169,29 @@ public static function createConnection($dsn, array $options = [])
      */
     protected function doFetch(array $ids)
     {
-        if ($ids) {
+        if (!$ids) {
+            return [];
+        }
+
+        $result = [];
+
+        if ($this->redis instanceof \Predis\Client && $this->redis->getConnection() instanceof ClusterInterface) {
             $values = $this->pipeline(function () use ($ids) {
                 foreach ($ids as $id) {
                     yield 'get' => [$id];
                 }
             });
-            foreach ($values as $id => $v) {
-                if ($v) {
-                    yield $id => parent::unserialize($v);
-                }
+        } else {
+            $values = array_combine($ids, $this->redis->mget($ids));
+        }
+
+        foreach ($values as $id => $v) {
+            if ($v) {
+                $result[$id] = parent::unserialize($v);
             }
         }
+
+        return $result;
     }
 
     /**