Consider adding a cleanup phase to schedule builds

## What problem does this solve or what need does it fill?
Schedule builds are expected to be relatively rare, but as of right now, most of the metadata used during the initial schedule builds remains allocated long after they're required ,even with the systems moved out.

## What solution would you like?
A separate part of the schedule lifecycle to clean up / deallocate the schedule build metadata.

## What alternative(s) have you considered?
Explicitly define the schedule lifecycle as an enum, which ensures that metadata from each phase in a schedule's lifecycle is properly cleaned up on moving to the next phase. This might be too brittle though if want to add/remove systems from the schedule at runtime.