Idea: Add an opt-in mode to act as a proxy without managing tokens

[TysonAndre]

This may be useful for apps that have many short-lived processes and would otherwise open and close a lot of connections (e.g. to APNS HTTP/2, or to GCM)

• HTTPS Handshakes to establish connections to gcm add time
• clients should not repeatedly open and close connections to APNs

This would be /proxied_push, which would behave like /push, except that

• The proposed endpoint would wait for all push responses to be sent up to a reasonable timeout.
• The redis db would add placeholders for removing tokens/subscriptions

The model of storing push tokens(i.e. delivery points) in redis has several inconveniences:

• Inconvenient to compute aggregate statistics such as how many subscribers use android, without a possibly inefficient/long-running repeated call to SCAN. Alternate push backends may help with that if implemented, by adding various indexes.