Extract node state into sans-I/O interface

[rustaceanrob]

Describe the enhancement

There is a fair amount of state tied in the Node struct that could be removed in favor of sans-I/O state machine. This state machine will just negotiate the construction of messages with Chain, and react to inbound events. The Node can simply alter this state and poll it for information.

Use case

For one it would be nice to test the top level Node structure with more scrutiny. Abstracting the state of the node also allows for future additions of a blocking interface.