Reintroduce user cache

This was required by avatars and was broken with https://github.com/owncloud/core/pull/16200

Fixes https://github.com/owncloud/core/issues/16942

lib/private/avatar.php

 	 * @return void
 	*/
 	public function set ($data) {
+
 		if($data instanceOf \OCP\IImage) {
 			$img = $data;
 			$data = $img->data();

lib/private/server.php

 		$this->registerService('AppHelper', function ($c) {
 			return new \OC\AppHelper();
 		});
-		$this->registerService('NullCache', function ($c) {
-			return new NullCache();
+		$this->registerService('UserCache', function ($c) {
+			return new Cache\File();
 		});
 		$this->registerService('MemCacheFactory', function (Server $c) {
 			$config = $c->getConfig();
 	 * @deprecated 8.1.0 use getMemCacheFactory to obtain a proper cache
 	 */
 	public function getCache() {
-		return $this->query('NullCache');
+		return $this->query('UserCache');
 	}
 
 	/**