This was necessary because Cargo recently deprecated the "overrides"
keyword in favor of "package", and Criterion changed its interface.
This commit also now lists more configurable keys, so if they aren't set
to their defaults the user will be able to find them. This includes the
roll key, which wasn't listed earlier and is different on Windows and
Mac. It does *not* include the respawn key because that key is already
shown when you die.
Currently we only do this when no players are in range of the chunk. We
also send the first client who posted the chunk a message indicating
that it's canceled, the hope being that this will be a performance win
in single player mode since you don't have to wait three seconds to
realize that the server won't generate the chunk for you.
We now check an atomic flag for every column sample in a chunk. We
could probably do this less frequently, but since it's a relaxed load it
has essentially no performance impact on Intel architectures.
