CI: optimizations (tracking issue)

[kaspar030]

Description

Optimizations:

• allow multiple parallel builds on ci.riot-os.org (so PR builds don't have to wait for "testing" builds)
  • enable on ci.riot-os.org
  • support multiple prioities (more than "normal" and "fasttracked") in murdock-ng
    • use priority queue instead of two regular queues: murdock: implement numeric priorities murdock-ng/murdock#18
    • allow setting priority through other means than "fasttracked" label
  • make murdock dwq backend prioritize PR jobs