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.

config.default.ini.php

@@ -52,3 +52,8 @@ username = ""
 ; The password for authentication. Insert this password when prompted for login.
 ; Use a strong password to prevent others from guessing your login!
 password = ""
+
+; --- Cache specific configuration ---------------------------------------------
+
+[SQLiteCache]
+file = "cache.sqlite"