use entirely local persistent data in modules

- eliminate qctx->hookdata and client->hookflags.
- use a memory pool to allocate data blobs in the filter-aaaa module,
  and associate them with the client address in a hash table
- instead of detaching the client in query_done(), mark it for deletion
  and then call ns_client_detach() from qctx_destroy(); this ensures
  that it will still exist when the QCTX_DESTROYED hook point is
  reached.

lib/ns/client.c

@@ -3047,7 +3047,6 @@ client_create(ns_clientmgr_t *manager, ns_client_t **clientp) {
 	ISC_QLINK_INIT(client, ilink);
 	client->keytag = NULL;
 	client->keytag_len = 0;
-	memset(client->hookflags, 0, sizeof(client->hookflags));
 
 	/*
 	 * We call the init routines for the various kinds of client here,