The approach of the code was rather hacky, simulating some packets
arriving from upstream and making the module stack CONSUME that.
Instead we take a direct approach now: use the simplified validator API
and then insert into cache directly.

One effect is improved performance, and consequently roughly halving
the lag which happens when prefill module invokes this.
(With root zone the lag goes down to 0.1 s from over 0.2 s,
 on my relatively fast CPU.  Fortunately it's just once a day.)