Don't store current-session side effects in `OnDiskCache`
This PR is a series of related cleanups to `OnDiskCache`, which is responsible for loading query return values (and side effects) that were serialized during the previous incremental-compilation session.
The primary change is to move the `current_side_effects` field out of OnDiskCache and into QuerySystem. That field was awkward because it was the only part of OnDiskCache state related to serializing the *current* compilation session, rather than loading values from the previous session.
The other commits should hopefully be straightforward.
r? nnethercote (or compiler)
Jacob Pratt
39343f53fd