Introduce pdo driver for symfony cache

[datengraben]

This enables database based caching (mysql) via Symfony PDOAdapter for hosts where file based caching is not possible.

When a redis connection and pdo is enabled, pdo is used for caching and redis is used for caching the tags.

TODO

• [ ] although it works. The pdoAdapter member variable which holds the adapter instance bloats the Plugin/Cache interface (bc it's private). It should be moved in a separated holder type.