Reuse `checkout` nodes created by `trigger` service for processing `patchset` nodes

[pawiecz]

PR #342 introduces new service which processes patchset nodes. It works on the assumption that they are created externally (together with their parent checkout node).

This issue aims to track connecting patchset service to reuse already existing checkout nodes created by the trigger service.

[pawiecz]

Context: #342 (comment)