Do not scan for keys just get all the keys (with prefix)

Apparently scan leads to some issues sometimes on cluster. So just use
the get function to fetch the keys.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

lib/private/Memcache/Redis.php

@@ -79,12 +79,10 @@ class Redis extends Cache implements IMemcacheTTL {
 
 	public function clear($prefix = '') {
 		$prefix = $this->getNameSpace() . $prefix . '*';
-		$it = null;
-		self::$cache->setOption(\Redis::OPT_SCAN, \Redis::SCAN_RETRY);
-		while ($keys = self::$cache->scan($it, $prefix)) {
-			self::$cache->del($keys);
-		}
-		return true;
+		$keys = self::$cache->keys($prefix);
+		$deleted = self::$cache->del($keys);
+
+		return count($keys) === $deleted;
 	}
 
 	/**