Use cds_lfht for lock-free hashtables in dns_adb

Use cds_lfht for lock-free hashtables in dns_adb

Replace the read-write locked isc_hashmap with lock-free cds_lfht
hashtable and replace the singular LRU tables for ADB names and entries
with a per-thread LRU tables.  These changes allowed to remove all the
read-write locking on the names and entries tables.