Follow-up from "Rate Limiting"

The following discussion from isc-private/kea-premium!272 should be addressed:

• @tmark started a discussion: (+1 comment)

  There is a configuration issue that needs to be addressed, possibly postpone until the next release

  Changes made to subnets via config back end or subnet_cmds would not be seen until the next reconfigure or reload

  DDNS-tuning addresses those as follows:

  1. For CB changes, it implements cb[46]_update callbacks - These look at audit entries to see if any subnets have changed, and if so updates the expression cache
  2. For subnet-cmd changes, it keeps track of the last time the expression cache was flushed, and the modification time of the subnet. If the latter is newer than it cache is flushed.