Very short missions can break the server state machine

[timhutton]

When this happens the clients must be restarted.

[DaveyBiggers]

For example, to reproduce, create a mission where the agent quits when reaching a goal, and set the agent's initial position to be on that goal square.

Why it goes wrong:

1. The server waits until all clients are running before it starts running itself.
2. Once the server is running, it listens out for mission-ended messages from the clients.
3. Once it gets one, it ends the mission (if ServerQuitWhenAnyAgentQuits is specified, eg.)

In between 1) and 2), the client can finish the mission and send the mission-ended message, and the server will miss it. In such cases, the server may never declare the mission over, and the Minecraft instance may need to be restarted.