899 add sidekiq timeout job

Shall fix #899 by integrating a timeout for those long-running jobs.
Currently the timeout defaults to 6 hours, which should be long enough, even for those rather large ontologies (or ontologies which import a lot of other ontologies).

Also includes specs.

Currently the timeout-worker just checks the current state of the version, and adjusts it, if it is not a terminal (`'failed'`, `'done'`) state.
Personally, I'd prefer `!%w(failed done).include?(ontology_version.state)`.
That's nothing that must be changed for the merge, though.