SimplePush: allow the client retrieve the full message body using {pushEndpoint,version} as a key

The idea is highlighted here: http://aerogear-dev.1069024.n5.nabble.com/aerogear-dev-Beyond-SimplePush-s-simplicity-tp9511.html

Because SimplePush protocol allows to transfer just incremental versions, we are not able to deliver any interesting message.

UnifiedPush Server could be a correct place where we unify and shield our users from this limitation:

My idea is to store the message on UPS under the SimplePush endpoint URL. Once the message with version reaches the client, he would contact UPS to retrieve this message under a key ( pushEndpoint, version ).

The messages should be removed once retrieved by a client.
The messages could have default built-in TTL to allow periodic cleanup.