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)

This is kind of overkill (also throwing processes with threads at this
particular cell), but it does in my testing save something like half a minute.

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)

This is kind of overkill (also throwing processes with threads at this
particular cell), but it does in my testing save something like half a minute.