deploy: garbage collection story

[bobheadxi]

Inertia's garbage collection right now is a weird mish-mash of "ill destroy assets whenever i feel like it". For example, Prune(), PruneAll() gets called at random places, and we dont have a good way of enforcing what gets deleted.

I'm thinking we introduce a package inertia/inertiad/daemon/gc, that is leveraged as a persistent goroutine to replace deployment.Watch(), to be dedicated to garbage collection, and in doing so make sure that no other part of the daemon deletes assets.

This will take a bit of planning to work out the best way to pull this off, and how to integrate it with work on #293 and #578