Do not create slots that exist already

Today couchdb knows everything that fully describes a slot. In the main job we should check if the slot we are trying to create does not already exist.

• We should decide what identifies a slot. Clearly the list of platforms, projects and their versions, but maybe also the versions of some of the tools? (e.g. consider a build failing because of issue in checkout logic)
• We should have a way to query couchdb for the existence of a slot. Iterating over all slots or something hash based?