[Support#12071] [RT#46548] Output stale/expired data with 'rndc dumpdb'

Now that there are ways to serve stale data from the cache, it would be good to be able to see with dumpdb what records there are that might end up being served, even though those records wouldn't show up in a regular dumpdb.

Update 2018-12-13: Stale data is actually already included in the dumpdb output. In addition to the stale RRsets it would be nice to also output for how long this stale data is still available in the cache. This needs to be determined by the configuration option 'serve-stale-max-ttl'.