MXS-1973 Support reverse DNS for client hostnames in MaxCtrl

May slow maxscale down when used. Only supported for "list sessions",
"show sessions" and "show session <id>".

maxutils/maxbase/include/maxbase/host.hh

@@ -108,4 +108,14 @@ inline bool operator!=(const Host& l, const Host& r)
 {
     return !(l == r);
 }
+
+/**
+ * Perform reverse DNS on an IP address. This may involve network communication so can be slow.
+ *
+ * @param ip IP to convert to hostname
+ * @param output Where to write the output. If operation fails, original IP is written.
+ * @return True on success
+ */
+bool reverse_dns(const std::string& ip, std::string* output);
+
 }