We need stricter locking before we can unload

Specifically, the "loose upgrades" behaviour that allows us to obtain an
exclusive right to load things while other requests are in progress (but
waiting on the exclusive lock for themselves) prevents us from treating
load & unload interchangeably: new things appearing is fine, but they do
*not* expect previously-present constants to vanish.

We can still use loose upgrades for unloading -- once someone has
decided to unload, they don't really care if someone else gets there
first -- it just needs to be tracked separately.

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