Minor changes are:
- Replace the "$RNDCCMD dumpdb" logic with "rndc_dumpdb" from
conf.sh.common (it does the same thing).
- Update a comment to match the grep calls below it (comment said the
rest should be expired, while the grep calls indicate that they
are still in the cache, the comment now explains why).
(cherry picked from commit 86a1bbfe28)
d784ab872c