in VRT_priv_task() we asserted that only one of ctx->req and ctx->bo is
set when not in vcl_pipe {}, but we also need to extend that assertion
to when ctx->method == 0 after vcl_pipe as finished because
VRT_priv_task() could be called from director resolution.

Being at it, I also noticed that our behavior in vcl_pipe {} is
inconsistent as, from the shard director perspective, it is a backend
method. So now, vcl_pipe {} is handled like vcl_backend_* {}.

We still need to make up our mind about #3329 / #3330 and depending on
the outcome we might need to touch some places again which were changed
in this commit.

Fixes #3361