Netcode + cleanup

- Renamed pva::sim::Timeline to pva::sim::Simulation
- New class pva::sim::Timeline representing a chronological
circular_buffer with useful iterators
- Now spawning an entity for clients who connect
- Smoother rendering. Client time never no longer snaps forward, time is
sped up when the client is far behind the ideal render time.
- Server rewind when receiving client input or (dis)connection events
Renamed "history" member of Simulation to "worlds"