+/* Threads:
+ * adns does not use any static modifiable state, so it
+ * is safe to call adns_init several times and then use the
+ * resulting adns_states concurrently.
+ * However, it is NOT safe to make simultaneous calls into
+ * adns using the same adns_state; a single adns_state must be used
+ * only by one thread at a time. You can solve this problem by
+ * having one adns_state per thread, or if that isn't feasible, you
+ * could maintain a pool of adns_states. Unfortunately neither of
+ * these approaches has optimal performance.
+ */
+