Implement PHP iterator based on the ZSCAN command (Redis 2.8).

This iterator allows to perform full iterations over the members of a sorted
set by wrapping the incremental nature of ZSCAN just like we did for SCAN:

  $client = new Predis\Client('tcp://127.0.0.1', ['profile' => '2.8']);
  $iterator = new Predis\Iterator\Scan\SortedSetIterator($client, "zset_key");

  foreach ($iterator as $member => $rank) {
      echo "$rank => $member" . PHP_EOL;
  }

Being ZSCAN closely related to SCAN, it is subject to the same behaviour,
see http://redis.io/commands/scan for reference.

This iterator implementation returns the member as key and the rank as value
since the rank is a float value which would be truncated when transforming
the iteration to an array (e.g. using iterator_to_array()). Luckily PHP
preserves the insertion order for named arrays members still result sorted.

lib/Predis/Iterator/Scan/SortedSetIterator.php

@@ -0,0 +1,58 @@
+<?php
+
+/*
+ * This file is part of the Predis package.
+ *
+ * (c) Daniele Alessandri <suppakilla@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Predis\Iterator\Scan;
+
+use Predis\ClientInterface;
+
+/**
+ * Abstracts the iteration of members stored in a sorted set
+ * by leveraging the ZSCAN command (Redis >= 2.8) wrapped in
+ * a fully-rewindable PHP iterator.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ * @link http://redis.io/commands/scan
+ */
+class SortedSetIterator extends AbstractScanIterator
+{
+    protected $key;
+
+    /**
+     * {@inheritdoc}
+     */
+    public function __construct(ClientInterface $client, $key, $match = null, $count = null)
+    {
+        $this->requiredCommand($client, 'ZSCAN');
+
+        parent::__construct($client, $match, $count);
+
+        $this->key = $key;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function executeScanCommand()
+    {
+        return $this->client->zscan($this->key, $this->cursor, $this->getScanOptions());
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function extractNext()
+    {
+        $element = array_shift($this->elements);
+
+        $this->position = $element[0];
+        $this->current = $element[1];
+    }
+}