Move RRSig loop to after DS loop to be more mindful of KeyTrap

Move RRSig loop to after DS loop to be more mindful of KeyTrap

In general we were mostly fine regarding KeyTrap, as we largely
fail after any invalid signature and only loop if a signature or
key required an unknown algorithm. Thus, addressing KeyTrap is
mostly an exercise in adding comments.

However, we did verify all DS hashes every time we went to verify
a single DNSKey RRSig, which is potentially some work, which we
fix here, leading to a nice simplification in `verify_rr_stream`.