Use queue polling instead of push for delivering data events

[alexrobin]

This makes it easier to handle consumers processing messages at different rate, as well as allow consumers to playback some messages.

However, a subscription mechanism has to be put it place and subscription info needs to be persisted between node restarts.

[alexrobin]

Solution: New Java 9 Flow interfaces will be used to implement push-based event distribution with optional back pressure from consumers

[alexrobin]

The new EventBus implementation in v2.0 exposes flow control capabilities offered by the Java 9 Flow API.