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.

bridges/ElloBridge.php

@@ -121,8 +121,8 @@ class ElloBridge extends BridgeAbstract {
 
 	private function getAPIKey() {
 		$cache = Cache::create(Configuration::getConfig('cache', 'type'));
-		$cache->setPath(PATH_CACHE);
-		$cache->setParameters(['key']);
+		$cache->setScope(get_called_class());
+		$cache->setKey(['key']);
 		$key = $cache->loadData();
 
 		if($key == null) {