[Lifecycle Events] add Minecraftserver instance api

[SpaceWalkerRS]

Similar to #46 but for the server, this PR adds a simple wrapper class for retrieving the current MinecraftServer instance. This is useful given that MinecraftServer::getInstance was only present in some versions.

The class looks as follows:

public class MinecraftServerInstance {

	public static MinecraftServer get() {
		return MinecraftServerAccess.getInstance();
	}
}

Internally this returns the instance if it is available, and otherwise throws an IllegalStateException. The instance is available from the start of the run method until the end of the shutdown method.

[SpaceWalkerRS]

Should there be other getters for integrated/dedicated servers?