Code review comment for lp://qastaging/~doanac/uci-engine/runner-refactor

Good points on the handle_request design, I'll try to make the best out of it.

Unfortunately, this conflicts with the design change required for the new autopkgtest, as explained since Malta and last summarized in https://app.asana.com/0/8499154990155/14578745408886 , adt-run needs to run on the worker not on the testbed anymore.