[fuzz] Make router_target a bit easier for fuzzers to explore

It turns out (somewhat obviously) that expecting a fuzzer to
correctly build multiple signatures which verify against multiple
public keys in the same message was a bit too daunting, so we now
skip message signatures in routing messages.

We also take this opportunity to simplify the target itself somewhat,
avoiding reading public keys over and over and instead generating
routes to all the public keys that appeared in messages while running.