Optimize memory layout of core structs

Reduce memory footprint by:

 - Reordering struct fields to minimize padding.
 - Using exact-sized atomic types instead of *_least/*_fast variants
 - Downsizing integer fields where possible

Affected structs:

 - dns_name_t
 - dns_slabheader_t
 - dns_rdata_t
 - qpcnode_t
 - qpznode_t

lib/dns/qpcache.c

@@ -169,9 +169,10 @@ struct qpcnode {
 	unsigned int nsec	: 2; /*%< range is 0..3 */
 	uint8_t			: 0;
 
+	uint16_t locknum;
+
 	isc_refcount_t references;
 	isc_refcount_t erefs;
-	uint16_t locknum;
 	void *data;
 
 	/*%