eagerly consider outputs as not needing deletion during output
registration rather than only doing so after registration. not
waiting for registration to succeed may keep store paths alive
in the file system if registration fails for some reason; that
seem preferrable to the possibility of having another instance
of this bug. since we only leave *good* outputs around there's
not much to worry about except maybe bit of wasted disk space.
fixes#883
Change-Id: I8c22c92e39b9e203f1061278f86cde19dc4474a4
eldritch horrors
4ef56601b5