Refactor instance communication

Change Instance class to be a subclass of Link and refactor the
communication to and establisment of instances to use a virtual
connection between the slave and the instance.  This is simplifies the
implementation of plugin communication for instances.

Replace InstanceRequest with a forwardTo option to Request which does
the task of forwarding in a much cleaner way.

test/integration/clusterio.js

@@ -63,7 +63,6 @@ describe("Integration of Clusterio", function() {
 			masterAuthToken: "invalid",
 			publicIP: "invalid",
 		});
-		testSlave.instances = await testSlave.findInstances();
 		testSlaveConnection = new master._SlaveConnection({ agent: "test", version, name: "slave", id: 4}, slaveServer);
 		master._slaveConnections.set(4, testSlaveConnection);
 	});