Ruby: request processing in multiple threads.

This closes #482 issue on GitHub.

src/nxt_main_process.c

@@ -242,6 +242,11 @@ static nxt_conf_map_t  nxt_ruby_app_conf[] = {
         NXT_CONF_MAP_STR,
         offsetof(nxt_common_app_conf_t, u.ruby.script),
     },
+    {
+        nxt_string("threads"),
+        NXT_CONF_MAP_INT32,
+        offsetof(nxt_common_app_conf_t, u.ruby.threads),
+    },
 };