For ban statistics, we updated VSC_C_main directly, so if we raced
Pool_Sumstat(), that could undo our changes.

This patch fixes statistics by using the per-worker statistics
cache except for the following remaining corner cases:

* bans_persisted_* counters receive absolut updates, which does
  not seem to fit the incremental updates via the per-worker stats.

  I've kept these cases untouched and marked with comments. Worst
  that should happen here are temporary inconsistencies until the
  next absolute update.

* For BAN_Reload(), my understanding is that it should only
  happen during init, so we continue to update VSC_C_main
  directly.

* For bans via the cli, we would need to grab the wstat lock,
  which, at the moment, is private to the worker implementation.

  Until we make a change here, we could miss a ban increment
  from the cli.

* for VCL bans from vcl_init / fini, we do not have access
  to the worker struct at the moment, so for now we also
  accept an inconsistency here.

Fixes #2716 for relevant cases