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

We want an option to dump, not just the stale/expired data, but the really really expired data.

The use case for this is that currently there is still no way to find out how much of cache is expired and awaiting clean-up without getting a core dump and analysing it. Feature request #101 (closed) tackles the stale case (for use by the serve-stale feature), but doesn't touch the fully-expired case.

Support (and customers) do, several times a year, encounter edge cases resolver query patterns instances where it appears that there are problems with caches holding on to expired RRsets for much longer than anticipated.

I do not think that this should be the default behaviour for rndc dumpdb -all - but an addition option -expired would make this content accessible to mere mortals as opposed to core dump analysts.