a simpler design for EventStore

@mathiasverraes I think @henrikbjorn is right here. You already introduced the DBAL dependency, so why not use it? Instead of a `MySqlPersistence`, there should be a `DoctrinePersistence`. This would make it possible to use an SQLite in-memory db for testing, which is a saviour once the functional tests reach a given size and complexity. Or somone has to use a different DB vendor because of business decisions.