consolidate fibonacci hashing and support case-insensitive hash tables

Fibonacci hashing was implemented in four separate places (rbt.c, rbtdb.c, resolver.c, zone.c). This commit combines them into a single implementation. The hash_32() function is now replaced with isc_hash_bits32().

This also adds an option to isc_ht_init() to specify whether keys in the hash table are case-sensitive or case-insensitive.