This commit does too many things, sorry, but stuff was all tangled up:
- clean up a bit (imports, unused variables)
- map to PDU with one query instead of in loop (each query gets all anyway)
- start fetching old constants (for comparison) in the background
- add qm layer to dict holding old constants (see issue #49)

Addresses https://git.xfel.eu/gitlab/detectors/pycalibration/merge_requests/464#note_228864

- trailing whitespace
- unused variables
- unused imports
- misc formatting