[cache] Directly implement CacheInterface in FileCache

The function 'prepare' previously implemented in CacheAbstract
is specifically required for FileCache and thus belongs to FileCache.
Since this change removes all code from CacheAbstract, it can be
removed completely.

lib/BridgeAbstract.php

@@ -196,7 +196,7 @@ abstract class BridgeAbstract implements BridgeInterface {
 		return static::URI;
 	}
 
-	public function setCache(\CacheAbstract $cache){
+	public function setCache(\CacheInterface $cache){
 		$this->cache = $cache;
 	}
 }