Sync chunks since last session

[Gjum]

• on connect, server sends list of each (up to ~1260) region's newest chunk ts - so we can ignore regions that are fully synced
• client finds region where its oldest chunk ts is older than server's newest chunk ts in that region
  • use file modification date? so we don't have to read the file's contents. must set explicitly because by default it's the newest chunk ts
• if a region is too old, client sends that region's chunk ts's (1024) to server; server responds with all newer chunk data of that region
• client only requests regions when not busy rendering

[Gjum]

reopening so we can implement this more intricate protocol

the reason is that the current protocol (everything after last connected timestamp) does not allow for stopping and resuming catchup when leaving the server or changing dimensions #17