Cached auto-slicing

[smartin015]

Is your feature request related to a problem? Please describe.

Currently STLs are auto sliced to gcode files when a print is attempted, regardless of whether they've been sliced before. This wastes time and compute if the file and slicer profile haven't changed.

Describe the solution you'd like

In CPQ, check the hash of the STL and the profile/slicer used before slicing. If the file has been sliced previously, use the cached file - otherwise, slice it anew.

With refactor to network queues to use IPFS, this cache could even be shared across printers on the network.

Describe alternatives you've considered

Upstream caching to PeerPrintService - this would improve PPS, but would mean that other slicers set up with CPQ wouldn't have caching.

Additional context

Emerged on thinking about inventory management flow (see #57)