nntp: resolve inboxes immediately on group listings
This prevents potential races between SIGHUP config reloads
while gigantic group listings are streaming, allowing us to
avoid many invalidation checks.
This also reduces send(2) syscalls and avoid Perl internal pad
allocations in a few places where it's not beneficial. There
might be a slight (0.5%) speedup, but I'm not sure if that's
down to system noise, power/thermal management, or other users
on my VM.