Use a qp-trie for the zone table

This change makes the zone table lock-free for reads. Previously, the
zone table used a red-black tree, which is not thread safe, so the hot
read path acquired both the per-view mutex and the per-zonetable
rwlock. (The double locking was to fix to cleanup races on shutdown.)

One visible difference is that zones are not necessarily shut down
promptly: it depends on when the qp-trie garbage collector cleans up
the zone table. The `catz` system test checks several times that zones
have been deleted; the test now checks for zones to be removed from
the server configuration, instead of being fully shut down. The catz
test does not churn through enough zones to trigger a gc, so the zones
are not fully detached until the server exits.

After this change, it is still possible to improve the way we handle
changes to the zone table, for instance, batching changes, or better
compaction heuristics.

lib/dns/client.c

@@ -206,7 +206,8 @@ createview(isc_mem_t *mctx, dns_rdataclass_t rdclass, isc_loopmgr_t *loopmgr,
 	isc_result_t result;
 	dns_view_t *view = NULL;
 
-	result = dns_view_create(mctx, rdclass, DNS_CLIENTVIEW_NAME, &view);
+	result = dns_view_create(mctx, loopmgr, rdclass, DNS_CLIENTVIEW_NAME,
+				 &view);
 	if (result != ISC_R_SUCCESS) {
 		return (result);
 	}