refactor hook mechanism:

1, remove EventHook;
2, setup_hooks: could reference request dict;
3, teardown_hooks: could reference Response object.

tests/httpbin/hooks.yml

@@ -9,8 +9,10 @@
         url: /headers
         method: GET
     setup_hooks:
-        - setup_hook_add_kwargs
-        - setup_hook_remove_kwargs
+        - ${setup_hook_add_kwargs($request)}
+        - ${setup_hook_remove_kwargs($request)}
+    teardown_hooks:
+        - ${teardown_hook_sleep_N_secs($response, 1)}
     validate:
         - eq: ["status_code", 200]
         - eq: [content.headers.Host, "127.0.0.1:3458"]