switch to generic worker for all tasks

[bhearsum]

docker-worker is deprecated, and causes us numerous issues, eg: #582, lack of memory monitoring, and others.

We need to switch to generic-worker for all tasks.

Some (but maybe not all) things we need to sort out before we can do this are:

• clean corpus unable to fork sometimes on generic-worker/d2g #640
• Many random errors on CPU generic workers #630
• docker tasks on generic worker sometimes hit issues with caches #538