Refactor WebAPI Workspace

[joschrew]

The code for the WebAPI regarding the workspace-part needs some refactoring:

• add database:
  • mongo-db because it is needed for upcomming processor part anyway and because knowledge how to use it is "available" in the team
• store information about an uploaded workspace in the database:
  • path to folder where it is stored on disk
  • mets-file path (if not mets.xml)
  • additional things (resolve the issues I added to the repo)
• add and refactor tests