Cache Cleaning Diagnostic Information

1. A counter in cachestats (JSON/XML) and ++ Cache Statistics ++ (named.stats) for the number of nodes without data
2. A counter in cachestats (JSON/XML) and ++ Cache Statistics ++ (named.stats) for the number of deadnodes
3. A counter in cachestats (JSON/XML) and ++ Cache Statistics ++ (named.stats) for the number of times that the function lib/dns/cache.c:incremental_cleaning_action is called
4. A counter in cachestats (JSON/XML) and ++ Cache Statistics ++ (named.stats) for the number of times that the function lib/dns/cache.c:overmem_cleaning_action is called
5. A counter in cachestats (JSON/XML) and ++ Cache Statistics ++ (named.stats) for the number of times that the function lib/dns/rbtdb.c:overmem_purge is called
6. A counter in cachestats (JSON/XML) and ++ Cache Statistics ++ (named.stats) for the number of times that the function lib/dns/rbtdb.c:cleanup_dead_nodes is called

Additional logging at DNS_LOGCATEGORY_DATABASE, DNS_LOGMODULE_CACHE, ISC_LOG_DEBUG(1) in the following functions:

1. lib/dns/rbtdb.c:overmem_purge - log node name (local name plus tree origin?) purged; log mctx in_use delta for both heap and tree after the purge
2. lib/dns/rbtdb.c:cleanup_dead_nodes - log bucketnum; log number of nodes purged; log mctx in_use delta for both heap and tree after the purge

Noting for the record that we already have CacheNodes/"cache database nodes" giving us the total node count.

Also noting that counts of fully expired and fully ancient nodes would be nice, but there aren't usually code events marking a node's transition from one group to another, so that will have to be something left for core dumps or full database traversals.

Finally, if this is to be prepared as a patch, can it please also include adding an INSIST(0) in lib/dns/rootns.c:dns_root_checkhints() immediately following the logging of "unable to get root NS rrset from cache"?