[DBAL-685] Add servicename connection parameter to Oracle drivers

Currently Doctrine's Oracle drivers do not distinguish between `SID` / `SERVICE_NAME` and `DBNAME` connection parameters which do not necessarily have to be the same in every database environment setup. The `SID` is the unique name of the database instance to connect to and can be superseded by a more general `SERVICE_NAME` which can be configured to contain multiple database instances in a HA environment on server side. For each individual database environment setup to work we need to be able to specify different values for `SID` / `SERVICE_NAME` and `DBNAME`.
This PR adds an additonal `servicename` connection parameter which is used instead of `dbname` for the `SID` / `SERVICE_NAME` connection parameters **IF** given. I also updated the docs to cover this and another parameter `service` which was introduced in [DBAL-141](http://www.doctrine-project.org/jira/browse/DBAL-141) but never got documented.