Define separate hash for rpmdb header check cache

Define separate hash for rpmdb header check cache

- This is really private to rpmdb.c (other than being exposed in
  the struct definition in backend/dbi.h, sigh), let it live its
  own life there.
- No functional changes here, just cleaning up a bit for next steps.
  OTOH we could now cache the actual result, not just success... but
  leaving that to another time.