Author: Alex Rousskov <rousskov@measurement-factory.com>

Author: Alex Rousskov <rousskov@measurement-factory.com>
Fix IP/FQDN cache accounting to avoid idle caches on busy servers.

When maintaining the IP/FQDN cache size, use the number of entries in the cache
rather than the number of allocated and not freed MEM_IPCACHE_ENTRY and
MEM_FQDNCACHE_ENTRY objects. These objects are used outside the cache
for DNS queries. If queries leak (or perhaps when there are just a lot of them),
the memory-pool-based count overestimates the cache size, sometimes to
such a degree that the cache remains nearly empty despite lots of misses.

Use memory-pool-based counter to estimate cache size also violates IP/FQDN cache
encapsulation boundaries because it effectively prevents others from using
the same memory pool.