Extend event replay to include hot/cold switchover

[daviddawson]

Currently, you don't know when the hot/ cold switch happens from the client point of view.

It would be useful to know this in many circumstances during replay.

For example, you have a view that is rebuilding after failure or new deploy, you want to keep that view out of service until it is up to date with the cold data being replayed. The only way to know that is if the event replay protocol includes a specific step to indicate that it has finished playing cold data, and is now switching to hot.

Discussion needed on whether this should be done implicitly by photon using the current API, or we should implement a specific event replay API with these extra semantics. I'm leaning towards the latter.