Use Doctrine DBAL

[chriskapp]

We could think about replacing our internal database classes with Doctrine DBAL. Therefor we only need to implement a wrapper-class which implements the ConnectionInterface and uses the DBAL connection. Currently we support the following features, which are not available at doctrine DBAL:

• Bulk insert
  There is no possibility to trigger a Bulk-Insert i.e. INSERT INTO (...) VALUES (...), (...), etc. There are multiple open PRs s. Bulk inserts doctrine/dbal#2762. As workaround we could simply trigger each query alone. We should check the performance impact of this.

• Upsert
  There is no way to trigger an upsert. An upsert allows you to update a row if the PK exists or create a new row. There is the following PR s. DBAL-163: Upsert support in DBAL doctrine/dbal#1320 As workaorund we could trigger two queries.

• Rename columns
  Not supported s. https://github.com/doctrine/dbal/blob/2.10.x/lib/Doctrine/DBAL/Schema/Table.php#L311 As workaround we could implement this manually.

  We should also adjust our API, so that we have two methods, one to rename a column and a second to change the data-type of a column. Currently we have only one method since MYSQL allows you to execute this in one ALTER query but Postgres, Oracle and MSSQL use two separate queries, which makes also more sense.

• Least Expression
  Currently not supported but this would be a good PR. We could use the following workaround: