Refactor the dns_name API to use ISC_THREAD_LOCAL

Previously, the dns_name API used isc_thread_key API for TLS, which is
fairly complicated and requires initialization of memory contexts, etc.
This part of code was refactored to use a ISC_THREAD_LOCAL pointer which
greatly simplifies the whole code related to storing TLS variables.

bin/named/main.c

@@ -1329,8 +1329,6 @@ cleanup(void) {
 	dlz_dlopen_clear();
 #endif
 
-	dns_name_destroy();
-
 	isc_log_write(named_g_lctx, NAMED_LOGCATEGORY_GENERAL,
 		      NAMED_LOGMODULE_MAIN,
 		      ISC_LOG_NOTICE, "exiting");