Same thing as for AGIPD.  Proposing to save this alongside the module mapping (if we're creating
YAML files per module anyway).  Note that I had to add "cap" and "qm" layers to old_const and
old_mdata; there's probably only one element in each, so the dictionary is overkill, but as we loop
over both, this would otherwise break if we ever have more than one of either...

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