Add helper methods to do safe partition reassignments

[wvanbergen]

@kvs @mkobetic

[mkobetic]

basic ruby question probably, but why do we have to convert to Sets here but get away with comparing things like replicas == replicas.uniq above? I assume these are all intended to be set operations basically.

[wvanbergen]

replicas.uniq will remove duplicate entries but leaves the order alone, which means that the equation holds if all the entries in the array are unique. We could also do Set.new(replicas).length == replicas.length to get the same result.

[mkobetic]

Ah ok, I didn't realize uniq will preserve order. Makes sense.

[wvanbergen]

cc @eapache because you were interested in the reassignment problem as well.

[eapache]

should this default to 3?

[mkobetic]

Hm, I forgot about nils. Is it worth building in a preference for filling in nils, before dropping replicas? Maybe adding the compact check will do most of that already.

[kvs]

Dunno if the new stuff I committed to rebalance.rb is useful here. It starts by filling up the nil-slots, then does something that's essentially identical to what's here.