Move trace-related logic from Build model to a separated class

As noticed by @rspeicher here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3447#note_4889541, we should think on moving the trace-related logic to a separated class like BuildTrace. Now we have a lot of it in the Build model.

Let's think how we could do this (I even have some ideas and I can prepare a proof of concept, but after 8.7 will be released).