Allow optional top level properties in messages

Add optional arguments to Request and Event to control which properties
are required to be present in the payload.  This allows gracefully
extending the protocol, reducing overhead for optional data, and leaving
properties out for additional behaviour.

test/lib/link/messages.js

@@ -17,11 +17,15 @@ describe("lib/link/messages", function() {
 		let testRequest = new libLink.Request({
 			type: "test",
 			links: ["source-target"],
+			requestRequired: ["test"],
 			requestProperties: {
 				"test": { type: "string" },
+				"optional": { type: "number" },
 			},
+			responseRequired: ["test"],
 			responseProperties: {
 				"test": { type: "string" },
+				"optional": { type: "number" },
 			},
 		});
 
@@ -127,8 +131,10 @@ describe("lib/link/messages", function() {
 		let testEvent = new libLink.Event({
 			type: "test",
 			links: ["source-target"],
+			eventRequired: ["test"],
 			eventProperties: {
 				"test": { type: "string" },
+				"optional": { type: "number" },
 			},
 		});