Add server checks for block placing

[Unarelith]

The server should check that:

• A placed block won't replace another one
• Blocks can't be placed inside a player

The client-side check made to fix #32 only checks the current player and the state of the world the client has. The server should check other players and its own state of the world (which is the real one). For example if two clients try to place a block at the same time, the server should only allow the first block placement packet it receives.

Actually, the client should also check for other players but that's another issue.