Store MergeWorker JID on merge request, and clean up stuck merges
https://gitlab.com/gitlab-org/gitlab-ce/issues/17709 / https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10207 do this for imports.
For merging MRs, we only unlock after a day: https://gitlab.com/gitlab-org/gitlab-ce/blob/v9.1.0-rc5/app/models/merge_request.rb#L649-653 That means if the worker starts and fails for some reason, but the failure isn't used to unlock the MR, we can't merge it.
We could use a similar approach to imports and unlock these if there's no running JID after a much shorter period (say, two hours).