Added support for server_version connection parameter

Doctrine DBAL 2.5 will support platform version autodetection, additionally it allows specifying serverVersion connection parameter:

> which the user can pass to skip platform autodetection and define the desired platform version straight away. This is also required for drivers that cannot return the database server version without an additional query (performance reasons).
> https://github.com/doctrine/dbal/pull/487

In symfony it can be useful eg. when warming up cache during build, so you don't need the db connection, but dbal 2.5 will try to autodetect server version anyway eg. during entity class metadata loading, then it will throw an exception because of unavailable connection. server_version parameter can be used to skip the autodetection.
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