agonized over this one for a while. The netty channel groups had to go
(enormous forced default sizes for even tiny, barely mutated sets)
and I think that is handled fine, but unsure on the concurrency handling
of its replacement. Since we are given a fixed set of < a few hundred
in size, that after initialization we only remove from, there should be
_something_. I am considering having the set itself be immutable, but to
represent their completion state with an atomic bit field. This would
encapsulate the idea that we know there is a limited world of possible
members and that we can map to them. The mapping could be something
trivial like the order of the set, but it could certainly be done
more efficiently.