ranked rrarray: don't merge RRSIGs of different types

For example, kr_rrmap_add has been relying on sets covering just a
single type (for years).  It's used by cache and there it's required to
store types separately.

It seems all other parts of code should be just OK with this.
6 jobs for rrsig-rank in 80 minutes and 36 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
canceled build:linux:amd64 #72890
linux docker amd64

passed build:linux:amd64 #11768
docker linux amd64

00:00:35

 
  Test
passed deckard:linux:amd64 #11770
docker linux amd64

00:00:39

passed respdiff:linux:amd64 #11772
docker linux amd64

01:19:57

passed test:linux:amd64 #11769
docker linux amd64

00:00:33

canceled respdiff:linux:amd64 #11771
docker linux amd64 allowed to fail

00:00:10