Extract `Node` interface from existing code

[tzdybal]

• Rename node.Node to node.FullNode
• Create node.Node interface by extracting all existing public functions of node.FullNode
• Create node.newFullNode function and move node.NewNode functionality there
• Modify node.NewNode to just call node.newFullNode (this will be changed later, see: Create stub of LightNode implementation #651).
• Consider moving noode.FullNode to node/full_node.go

[S1nus]

#646

[tzdybal]

We decided not to proceed with this approach.