Specify behavior if SSE connection drops

[jwise-mfg]

We may need a "re-connect" interface.

[matthewGEA]

In #162, I propose a dedicated GET /subscriptions/{subscriptionID}/stream resource, to allow the client to connect/disconnect without needing to recreate the subscription.

Decoupling registration from the actual stream also allows the client to "slipstream" items into the subscription without having to delete the existing subscription and recreating.

[jwise-mfg]

I believe this will be addressed with the Subscription management changes Aron's team is making.