incrementally clean up old RPZ records during updates

After an RPZ zone is updated via zone transfer, the RPZ summary database is updated, inserting the newly added names in the policy zone and deleting the newly removed ones. The first part of this was quantized so it would not run too long and starve other tasks during large updates, but the second part was not quantized, so that an update in which a large number of records were deleted could cause named to become briefly unresponsive.

Closes #1447 (closed)