NxsGetter.mirrors.next has surprising and possibly bug-inducing
side-effects. Calls to the function change the state of NxsGetter by altering
_buf and _write_to_fp. I inserted some comments to warn about this and
rearranged a call to mirrors.next in NxsGetter.get to fix a problem caused by
the side-effect.

This is all because of previous changes that were made to reduce the total
number of curl handles floating around. It might not be worth it.