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.

lib/contents.php

@@ -46,11 +46,11 @@ function getContents($url, $header = array(), $opts = array()){
 
 	// Initialize cache
 	$cache = Cache::create(Configuration::getConfig('cache', 'type'));
-	$cache->setPath(PATH_CACHE . 'server/');
+	$cache->setScope('server');
 	$cache->purgeCache(86400); // 24 hours (forced)
 
 	$params = [$url];
-	$cache->setParameters($params);
+	$cache->setKey($params);
 
 	// Use file_get_contents if in CLI mode with no root certificates defined
 	if(php_sapi_name() === 'cli' && empty(ini_get('curl.cainfo'))) {
@@ -271,11 +271,11 @@ function getSimpleHTMLDOMCached($url,
 
 	// Initialize cache
 	$cache = Cache::create(Configuration::getConfig('cache', 'type'));
-	$cache->setPath(PATH_CACHE . 'pages/');
+	$cache->setScope('pages');
 	$cache->purgeCache(86400); // 24 hours (forced)
 
 	$params = [$url];
-	$cache->setParameters($params);
+	$cache->setKey($params);
 
 	// Determine if cached file is within duration
 	$time = $cache->getTime();