Add API hook after the bus has started but before the dispatch queue starts delivering messages

[Mendel Monteiro-Beckerman]

Currently there is no way for a HostInitializer to initialise itself with in a initial state before receiving messages. We have found this to be a common pattern and also a common source of bugs resulting from the inherent race condition (messages received before initial state load).

I propose adding an API hook at a point where the peer has successfully subscribed to the bus and has started receiving messages but is not yet forwarding them to the handlers.