trust anchors: nitpicks

- Remove abstraction of the refresh callback, as we always use the same
  function.  This way the code is significantly easier to understand.
- Add a couple more comments.
3 jobs for <span class="ref-name">ta-multiple</span> in 1 minute and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #2813
docker linux amd64
build:linux:amd64

00:00:30

 
  Test
passed #2815
docker linux amd64
deckard:linux:amd64

00:01:07

passed #2814
docker linux amd64
test:linux:amd64

00:00:25