Use BSONHandlers everywhere

[felixgabler]

We can take a look at migrating everything to use BSONHandlers with automatic serialization to BSON.
If I remember correctly, some keys still need to be changed to camelCase. Don't forget the migrations.

Files that still use BSONDocumentWriter or BSONDocumentReader:

• User.scala
• Job.scala
• JobClusterData.scala
• JobEvent.scala
• JobEventLog.scala
• StatisticsObject.scala
• ToolStatistic.scala
• UserConfig.scala
• UserToken.scala