Server should remember what blocks clients have loaded

[bfops]

The server needs to send terrain updates to clients (e.g. for https://github.com/bfops/playform/issues/124 and https://github.com/bfops/playform/issues/67), so each terrain block should have a record of which clients hold it. We also need to have the clients tell servers when they don't need a block anymore.

The other option I see is to more pervasively make the assumption that all clients are always loading blocks within some radius of themselves, but that seems restrictive (not all clients are necessarily strictly there for the sake of watching and controlling exactly one player).