Explore state machine execution model

[iand]

Example is in examples/statemachine, run demo with go run .

Only the simplest happy path is implemented to demonstrate the idea.

Key points of approach

• Queries are executed by a hierarchical state machine (with thanks to rust-libp2p design)
• KademliaHandler runs the event loop and query state machine
• IPFS-specific logic lives in IpfsDht
• KademliaHandler doesn't know anything about IPFS messages, just knows how to perform a query and send a MinKadRequestMessage to each node.
• FindNodeRequest and FindNodeResponse are the messages that the IpfsDht knows about, in reality they would be the protobuf messages
• The state machine and event model provides a lot of extensibility and possibilities for notifying multiple components about changes such as new peers being discovered or found to be unresponsive

[iand]

Refactored to be a standalone example. Merging as discussed in Paris.