First event flow approach

 * There are 4 states: client_connect, client, server_accept, server.
 * There are many operations: send updates, rcv updates, send join, rcv join, etc..
 * The daemon jumps from one state to other and to one operation to another.

By now, nft-sync does nothing except printing function messages and
exchange a JOIN message to know which instance is older/newer.

More work in near future.