multiplayer stage 8: state correction and surviving a dropped connection

Two things were still missing once a session ran for a while: a peer that had
drifted apart from the authority had no way back short of restarting, and a
connection that blinked cost the player their seat and handed their vehicle to
the AI while the other driver was still sitting in it.

- a client that disagrees with the authoritative digest for 120 steps in a row
  asks to be put back in line, and the server answers with a fresh snapshot.
  There is a cooldown between requests, so a correction that needs a moment to
  take hold does not turn into a stream of them
- the server hands out a session token with its hello and remembers it. Present
  it again on the next connection and you come back as the same peer, with the
  same id
- a lost connection no longer removes the peer from its crews: the seat is held
  for a minute. Come back in time and nothing changed; stay away and the seat
  is given up properly, handing the vehicle back to the AI only when that
  leaves the crew empty and the AI was driving it in the first place
- after applying a snapshot a peer that finds itself on a crew walks into that
  cab, which is what makes a reconnect land the player back where they were
- the vehicle roster is sent to a joining peer right before its snapshot, so
  the crew section resolves to names instead of waiting for the next broadcast

network/network.h

@@ -124,6 +124,8 @@ namespace network
 		void send_commands(command_queue::commands_map commands);
 		// tells the server the scenario is loaded and a snapshot can be applied
 		void send_ready();
+		// our world has drifted too far to carry on; ask to be put back in line
+		void send_resync_request(uint64_t tick, uint64_t state_hash);
 		// lobby requests; the server is the one that decides
 		void send_claim(NetworkEntityId entity_id);
 		void send_leave(NetworkEntityId entity_id);