Race condition in server.go could cause panic on server close

[Sandertv]

In server.go, the channel to which newly joined players are added, is closed when the server is closed here: https://github.com/df-mc/dragonfly/blob/master/server/server.go#L336. When a player joins, https://github.com/df-mc/dragonfly/blob/master/server/server.go#L339 is called (note the go), which then, on another goroutine, submits a newly created player to the channel here: https://github.com/df-mc/dragonfly/blob/master/server/server.go#L381.

If a player is in the process of spawning and the server is closed, golang will panic because dragonfly will try to insert a value into a closed channel.