Support delay message? about bark-server

[mritd]

There seems to be no relevant description in the APNS official documents. If APNS does not support this feature, the bark server will not implement it.

Because bark server may run in embedded devices, custom features will occupy higher resources and bring more complex problems; the goal of bark server is to serve as a standard api server to provide reliable push APIs for bark app.

The custom scheduling scheme should be implemented by the caller instead of relying on the bark server.