resolved: rework how and when the number of answer RRs to cache is determined

resolved: rework how and when the number of answer RRs to cache is determined

Instead of figuring out how many RRs to cache right before we do so,
determine this at the time we install the answer RRs, so that we can
still alter this as we manipulate the answer during validation.

The primary purpose of this is to pave the way so that we can drop
unsigned RRsets from the answer and invalidate the number of RRs to
cache at the same time.