Reduce the default RBT hash table size to 16 entries (4 bits)

The hash table rework MRs (!3865, !3871) increased the default RBT hash
table size from 64 to 65,536 entries (for 64-bit architectures, that is
512 bytes before vs. 524,288 bytes after).  This works fine for RBTs
used for cache databases, but since three separate RBT databases are
created for every zone loaded (RRs, NSEC, NSEC3), memory usage would
skyrocket when BIND 9 is used as an authoritative DNS server with many
zones.

The default RBT hash table size before the rework was 64 entries, this
commit reduces it to 16 entries because our educated guess is that most
zones are just couple of entries (SOA, NS, A, AAAA, MX) and rehashing
small hash tables is actually cheap.  The rework we did in the previous
MRs tries to avoid growing the hash tables for big-to-huge caches where
growing the hash table comes at a price because the whole cache needs to
be locked.

lib/dns/rbt.c

@@ -59,7 +59,7 @@
 #define CHAIN_MAGIC	   ISC_MAGIC('0', '-', '0', '-')
 #define VALID_CHAIN(chain) ISC_MAGIC_VALID(chain, CHAIN_MAGIC)
 
-#define RBT_HASH_MIN_BITS   16
+#define RBT_HASH_MIN_BITS   4
 #define RBT_HASH_MAX_BITS   32
 #define RBT_HASH_OVERCOMMIT 3
 #define RBT_HASH_BUCKETSIZE 4096 /* FIXME: What would be a good value here? */