caches: Refactor the API (#1060)

- For consistency, functions should always return null on non-existing data.

- WordPressPluginUpdateBridge appears to have used its own cache instance in the past. Obviously not used anymore.

- Since $key can be anything, the cache implementation must ensure to assign the related data reliably; most commonly by serializing and hashing the key in an appropriate way.

- Even though the default path for storage is perfectly fine, some people may want to use a different location. This is an example how a cache implementation is responsible for its requirements.

actions/DisplayAction.php

@@ -86,9 +86,9 @@ class DisplayAction extends ActionAbstract {
 
 		// Initialize cache
 		$cache = Cache::create(Configuration::getConfig('cache', 'type'));
-		$cache->setPath(PATH_CACHE);
+		$cache->setScope('');
 		$cache->purgeCache(86400); // 24 hours
-		$cache->setParameters($cache_params);
+		$cache->setKey($cache_params);
 
 		$items = array();
 		$infos = array();