remove randomness in favor of a simpler 1 of N procedure

This deserves a bit of explanation, as the motivating issue explicitly
requested randomness. In short, it's hard to get randomness to compile
for `wasm32-unknown-unknown` because that is explicitly intended to be
as deterministic as practical. Additionally, even though it would never
be used for consensus purposes, it still felt offputting to intentionally
introduce randomness into a node's operations. Except, it wasn't really
random, either: it was a deterministic PRNG varying only in its state,
and getting the state to work right for that target would have required
initializing from a constant.

Given that it was a deterministic sequence anyway, it seemed much simpler
and more explicit to simply select one of each N messages instead of
attempting any kind of realistic randomness.